Patent Application
20050133297
The present invention relates to a rack assist type electric power steering apparatus in which a belt type power transmission mechanism is employed.
In the steering system of vehicles, so-called power steering apparatuses for assisting the steering operation by means of an external power source are widely used. Conventionally, vane type hydraulic pumps have been used as the power source of the power steering apparatuses. In many cases, the hydraulic pump is driven by the engine. However, this type of power steering apparatus suffers from large engine driving loss (as much as several or ten horsepower in the maximum load state) due to constant driving of the hydraulic pump. Therefore, it is difficult to use this type of power steering apparatus in mini-sized motor vehicle with a small engine displacement, and even in the case of vehicles with a relatively large displacement, a considerable decrease in gas mileage is inevitable.
In view of the above situation, an electric power steering apparatus (Electric Power Steering, which will be referred to as EPS) that uses an electric motor as the power source has been getting attention in these years as a solution for the above-described problems. The EPS has advantageous features, such as: it does not suffers from direct engine driving loss since an in-vehicle battery is used as the power source for the electric motor; a decrease in gas mileage can be avoided since the electric motor is activated only upon assisting steering; and electric control thereof is quite easy.
In the meanwhile, the current mainstream of the steering gear for vehicles is the rack and pinion type steering gear because of its high rigidity and light weight or other reasons. As an EPS for the rack and pinion type steering gear, a ball screw rack assist type EPS that drives a rack shaft by means of an electric ball screw mechanism is used as well as a column assist type EPS in which the electric motor is disposed on the column side so as to drive the steering shaft or the pinion itself. In the ball screw rack assist type EPS (which will be simply referred to as the rack assist type EPS), since the assisting force does not act on the engaging surfaces of the pinion and the rack, the contact surface pressure between those members, which can cause friction or deformation, is relatively small.
In the rack assist type EPS, the male thread groove of the ball screw shaft formed on the rack shaft engages with the female thread groove formed on the ball nut through a number of circulating balls (steel balls), and the ball nut is rotationally driven by an electric motor disposed coaxially or non-coaxially with the rack shaft, so that the rack shaft is moved in the axial direction. As a power transmission system between the electric motor and the ball nut in the no-coaxial rack assist type EPS, a gear system disclosed in Japanese Patent No. 3062852 or a belt system, which is advantageous in quietness and transmission efficiency, is used.
In conventional non-coaxial rack assist type EPSs using a belt, as described in Japanese Utility Model Application Laid-Open No. 60-122274 or Japanese Utility Model Publication 4-28538, the power transmission mechanism includes a drive pulley, a driven pulley and a belt looped around the two pulleys. It is important that an appropriate tension is given to the belt. When the drive pulley starts to rotate, a driving force is transmitted to the driven pulley (or the ball nut) by means of the belt, whereby a tensile force is exerted on the driving side web (the tensioned side web) of the belt due to drive resistance while the non-driving side web (the not-tensioned side web) is loosened. If this looseness exceeds a certain degree, slippage will occur between the belt and the pulleys in the case that the belt is a V-belt or a flat belt, or tooth skip will occur between the belt and the pulleys, so that power will not be transmitted appropriately and damages such as burning or tooth drop will occur in the belt.
A general method of giving a tension to the belt is to provide a tensioner mechanism in the belt housing thereby pressing a tension pulley against the non-tension side web with a biasing force of a coil spring or a hydraulic force. However, in the case of the power transmission mechanism of the EPS, since the rotation direction of the drive pulley changes depending on the steering direction changed by the driver, if the tension pulley is pressed against the tension side web, the tension pulley will be moved back by the tension of the tension side web to cause a delay in power transmission to the driven pulley. This causes the problem that steering assist cannot be effected smoothly. This disadvantage also occurs in the case that tensioners are provided on both sides of the belt since the tension pulleys are moved back due to the tension of the tension side web.
The present invention has been made in view of the above-described situation. An object of the present invention is to provide a rack assist type electric power steering apparatus in which tension adjustment of a belt in the power transmission mechanism can be carried out appropriately and easily.
To achieve the above object, according to a first aspect of the present invention, there is provided a rack assist type electric power steering apparatus comprising a ball screw mechanism including a ball nut having a female thread groove formed on its inner circumferential surface, a rack shaft disposed at the central axis of the ball nut and having a male thread groove that is opposed to said female thread groove, formed on its outer circumferential surface and a plurality of circulating balls disposed between said female thread groove and said male thread groove, an electric motor disposed non-coaxially with said rack shaft for driving said ball nut mechanism, power transmission means including a drive pulley rotating integrally with the shaft of the electric motor, a driven pulley rotating integrally with said ball nut and a belt looped around the drive pulley and the driven pulley, and a steering gear case for accommodating said power transmission means and said ball screw mechanism, wherein said electric power steering apparatus is provided with a motor flange fixed to an end portion of said electric motor and having a cylindrical shape to be fitted inside a motor holding bore formed in said steering gear case, and said motor flange rotatably supports said drive pulley and is adapted to rotate relative to said motor holding bore within a certain angle range, and the central axis of the motor flange is offset from the central axis of said electric motor.
According to the first aspect of the present invention, for example, after power transmission means is assembled, the motor flange is rotated while measuring the tension of the belt by means of a tension meter, and when a proper initial tension is attained, the motor flange is secured to the steering case.
In the electric power steering apparatus according to the first aspect of the present invention, a belt retaining flange may preferably be formed on at least one of said drive pulley and said driven pulley. In this case, axial displacement of the belt during running is regulated, and smooth power transmission can be realized.
In the electric power steering apparatus according to first aspect of the present invention, said motor flange may preferably be separated into two parts along the axial direction. With this feature, assembling of the drive pulley and the belt into the motor flange can be made easy.
In the electric power steering apparatus according to the first aspect of the present invention, said motor flange may preferably be provided with a belt accommodating portion for accommodating the belt in a folded state during an assembling operation. In this case, with use of a subassembly in which the drive pulley and the belt are assembled to the motor flange in advance, the operation required after attachment to the steering gear case is greatly simplified, and reduction of the number of assembling steps is realized.
In the electric power steering apparatus according to the first aspect of the present invention, said motor holding bore may preferably annexed with a belt passing portion for allowing the belt projecting from the belt accommodating portion to pass through. With this feature, when the motor flange is assembled to the steering gear case, the belt projecting from the motor flange will not interfere with the steering gear case.
In the electric power steering apparatus according to the present invention, the belt accommodated in said belt accommodating portion may preferably be folded in a state in which the belt is prevented from assuming a curvature smaller than a predetermined curvature by curvature regulation means. With this feature, break of reinforcing fibers of the belt due to bending with excessively small curvatures can be prevented.
In the electric power steering apparatus according to the first aspect of the present invention, it is preferable that a power transmission means accommodating portion for accommodating said power transmission means and a ball screw mechanism accommodating portion for accommodating said ball screw mechanism be isolated by isolation means, and said belt be supported by a closed rolling bearing. With this structure, lubricant for lubricating the ball screw mechanism or bearings will not attach to the belt of the power transmission means, and deterioration of the synthetic rubber constituting the belt due to swelling etc. can be prevented.
According to a second aspect of the present invention, there is provided a rack assist type electric power steering apparatus comprising a ball screw mechanism including a ball nut having a female thread groove formed on its inner circumferential surface, a rack shaft disposed at the central axis of the ball nut and having a male thread groove that is opposed to said female thread groove, formed on its outer circumferential surface and a plurality of circulating balls disposed between said female thread groove and said male thread groove, an electric motor disposed non-coaxially with said rack shaft for driving said ball screw mechanism, power transmission means including a drive pulley connected to the shaft of the electric motor, a driven pulley rotating integrally with said ball nut and a belt looped around the drive pulley and the driven pulley, and a steering gear case for accommodating said ball screw mechanism and said power transmission means, wherein said electric power steering apparatus is provided with tension adjusting means for adjusting the tension of said belt, and said tension adjusting means includes a pulley holder rotatably supporting said drive pulley and held in said steering gear case in a swingable manner and holder angle adjusting means for adjusting the swing angle of said pulley holder.
According to the second aspect of the present invention, by changing the swing angle of the pulley holder by the holder angle adjusting means, the distance between the center of the drive pulley and the center of the driven pulley can be changed to give a desired tension to the belt.
In the electric power steering apparatus according to the second aspect of the present invention, said holder angle adjusting means may preferably comprise a screw mechanism. In this case, the tension adjustment can be effected in a stepless manner finely, and displacement of the roller holder after tension adjustment is unlikely to occur.
In the electric power steering apparatus according to the second aspect of the present invention, the shaft of said electric motor and said drive pulley may preferably be connected by misalignment absorbing means. In this case, even if, for example, misalignment of the shaft of the electric motor and the drive pulley occurs upon tension adjustment, the misalignment is absorbed by the misalignment absorbing means. Therefore, the electric motor may be fixed to the steering gear case in advance.
In the electric power steering apparatus according to the second aspect of the present invention, said misalignment absorbing means may preferably be an Oldham coupling. In this case, even if misalignment of the shaft of the electric motor and the drive pulley occurs, power transmission from the electric motor to the drive pulley is achieved without a significant transmission loss.
In the electric power steering apparatus according to the second aspect of the present invention, a belt retaining flange may preferably be formed on at least one of said drive pulley and said driven pulley. In this case, axial displacement of the belt during running is regulated, and smooth power transmission can be realized.
In the electric power steering apparatus according to the second aspect of the present invention, said steering gear case may preferably include a belt housing for accommodating said power transmission means and a ball screw housing secured to said belt housing for accommodating said ball screw mechanism, and said driven pulley and said ball nut may be spline connected. In this case, the adjustment of the tension of the belt can be effected in the state in which the ball screw housing has not been secured, and the driven pulley and the ball nut can be connected easily upon securing the belt housing and the ball screw housing.
In the electric power steering apparatus according to the second aspect of the present invention, a power transmission means accommodating portion for accommodating said power transmission means and a ball screw mechanism accommodating portion for accommodating said ball screw mechanism may preferably be isolated by isolation means, and said belt may be supported by a closed rolling bearing. In this case, lubricant for lubricating the ball screw mechanism or bearings will not attach to the belt of the power transmission means, and deterioration of the synthetic rubber constituting the belt due to swelling etc. can be prevented.
According to a third aspect of the present invention, there is provided a rack assist type electric power steering apparatus comprising, a ball screw mechanism including a ball nut having a female thread groove formed on its inner circumferential surface, a rack shaft disposed at the central axis of the ball nut and having a male thread groove that is opposed to said female thread groove, formed on its outer circumferential surface and a plurality of circulating balls disposed between said female thread groove and said male thread groove, an electric motor disposed non-coaxially with said rack shaft for driving said ball nut mechanism, power transmission means including a drive pulley rotating integrally with the shaft of the electric motor, a driven pulley rotating integrally with said ball nut and a belt looped around the drive pulley and the driven pulley, and a steering gear case for accommodating said ball screw mechanism and said power transmission means, wherein said electric power steering apparatus further comprises tension adjusting means for adjusting the tension of said belt, and said steering gear case includes a belt housing for accommodating said power transmission means and a ball screw housing secured to said belt housing for accommodating said ball screw mechanism.
According to the third aspect of the present invention, in the state in which the ball screw housing is not secured, the tension of the belt is estimated based on the drive resistance of the drive pulley or the driven pulley while operating the tension adjusting means, and after completion of the tension adjustment, the belt housing and the ball screw housing are secured together to assemble the steering gear.
In the electric power steering apparatus according to the third aspect of the present invention, said driven pulley and said ball nut may preferably be spline connected. With this feature, the connection of the driven pulley and the ball nut can be achieved easily upon securing the belt housing and the ball screw housing together.
In the electric power steering apparatus according to the third aspect of the present invention, a belt retaining flange may preferably be formed on at leas one of said drive pulley and said driven pulley. With this feature, axial displacement of the belt during running is regulated, and smooth power transmission can be realized.
In the electric power steering apparatus according to the third aspect of the present invention, the tension of said belt may preferably be measured based on the running torque of said drive pulley or said driven pulley.
In the electric power steering apparatus according to the third aspect of the present invention, said tension adjusting means may be adapted to change the distance between the center of said drive pulley and the center of said driven pulley. With this feature, the tension of the belt is adjusted by changing the distance between the center of the pulleys by, for example, moving the drive pulley toward or away from the driven pulley using an eccentric shaft.
In the electric power steering apparatus according to according to the third aspect of the present invention, a power transmission means accommodating portion for accommodating said power transmission means and a ball screw mechanism accommodating portion for accommodating said ball screw mechanism may preferably be isolated by isolation means, and said belt may be supported by a closed rolling bearing. In this case, lubricant for lubricating the ball screw mechanism or bearings will not attach to the belt of the power transmission means, and deterioration of the synthetic rubber constituting the belt due to swelling etc. can be prevented.
According to a fourth aspect of the present invention, there is provided a rack assist type electric power steering apparatus comprising a ball screw mechanism including a ball nut having a female thread groove formed on its inner circumferential surface, a rack shaft disposed at the central axis of the ball nut and having a male thread groove that is opposed to said female thread groove, formed on its outer circumferential surface and a plurality of circulating balls disposed between said female thread groove and said male thread groove, an electric motor disposed non-coaxially with said rack shaft for driving said ball nut mechanism, power transmission means including a drive pulley rotating integrally with the shaft of the electric motor, a driven pulley rotating integrally with said ball nut and a belt looped around the drive pulley and the driven pulley, and a steering gear case for accommodating said ball screw mechanism and said power transmission means, wherein said electric power steering apparatus further comprises tension adjusting means for adjusting the tension of said belt, and said tension adjusting means includes a tension roller rotatably held by a roller holder disposed outside said belt, said tension roller being adapted to be moved back and forth relative to a web surface of said belt together with said roller holder, and a roller pressing means for pressing said tension roller against said belt.
According to the fourth aspect of the present invention, when the tension roller is pressed against the outer side of the belt, the wrapping angle of the belt on the pulleys is increased, so that the shear stress of the belt at the side of the pulley having the smaller diameter is reduced.
In the electric power steering apparatus according to the fourth aspect of the present invention, said roller holder may preferably be adapted to swing about a swing shaft as a supporting point. With this feature, by changing the positional relationship between the tension roller, the swing shaft and the roller pressing means, moving amount of the tension roller relative to the drive amount by the roller pressing means can be determined relatively freely.
In the electric power steering apparatus according to the fourth embodiment of the present invention, said roller pressing means may preferably be a screw mechanism. With this feature, the tension adjustment can be effected in a stepless manner finely, and displacement of the roller holder after tension adjustment is unlikely to occur.
In the electric power steering apparatus according to the fourth aspect of the present invention, it is preferable that in the state in which said power transmission means is assembled, a line connecting the center axis of said tension roller and said swing axis be substantially parallel to the side of the web of said belt that is not pressed by said tension roller. With this feature, the moving direction of the tension roller caused by swing of the roller holder becomes substantially perpendicular to the belt, and therefore the tension roller is pressed against the belt effectively with a relatively small swing amount.
In the electric power steering apparatus according to the fourth aspect of the present invention, it is preferable that belt retaining flanges for regulating axial displacement of said belt be formed at both ends of said tension roller, and said tension roller be movable within a predetermined movement range. With this feature, problems such as dislocation of the belt from the tension roller or meandering of the belt due to displacement of the roller holder are prevented.
In the electric power steering apparatus according to the fourth aspect of the present invention, it is preferable that said roller holder be a press formed part. With this feature, weight reduction of the roller holder is achieved and improvement in its productivity is realized.
In the electric power steering apparatus according to the fourth aspect of the present invention, it is preferable that a power transmission means accommodating portion for accommodating said power transmission means and a ball screw mechanism accommodating portion for accommodating said ball screw mechanism be isolated by isolation means, and said belt be supported by a closed rolling bearing. With this feature, bearings will not attach to the belt of the power transmission means, and deterioration of the synthetic rubber constituting the belt due to swelling etc. can be prevented.
As per the above, it is possible to give a proper tension to the belt reliably and easily without using a tensioner or the like, and smooth steering assist and prevention of damages of the timing belt can be realized while reducing the number of components.
In the following, embodiments of the present invention will be described with reference to the drawings.
Connected to the lower end of the lower steering shaft 9 is a steering gear 11 composed of a rack and pinion mechanism and a power assist mechanism etc. In
The outer hull of the power assist mechanism is composed of a belt housing 31 secured to the right side (in
On the lower portion of the belt housing 31, an electric motor 35 is attached. The belt housing 31 accommodates a drive pulley 37 fixed to the shaft 36 of the electric motor 35, a hollow driven pulley 39 connected to a ball nut 51 that will be described later and a timing belt 41 looped around the drive pulley 37 and the driven pulley 39, which constitute a power transmission mechanism. The drive pulley 37 is supported on a motor flange 71 and a flange cover 75 integral with it via prelubricated deep groove ball bearings 43 and 44 (which will be simply referred to as bearing hereinafter) as closed rolling bearings respectively. The driven pulley 39 is supported on the housings 21 and 31 via bearings 45 and 46 respectively. On both ends of the drive pulley 37, belt retaining flanges 48 and 49 are attached. In the case of this embodiment, as will be understood from
In the ball screw housing 33, a ball nut 51 is rotatably supported by means of a double row angular ball bearing 53. Reference sign 55 in
On the rack shaft 23, a male thread groove 61 is formed, while on the ball nut 51, a female thread groove 63 is formed, and a number of steel balls 65 serving as circulating balls are provided between the male thread groove 61 and the female thread groove 63. In addition, a circulation piece (not shown) for circulating the steel balls 65 is attached on the ball nut 51.
On the ball nut 51 side end portion of the driven pulley 39, a female spline 67 is formed, and a male spline 69 engaging the female spline 67 is formed on the ball nut 51. Thus, in the assembled state, the driven pulley 39 and the ball nut 51 rotate together.
At the end portion of the electric motor 35, a motor flange 71 having a substantially cylindrical shape is secured by bolts. The fitting portion 72 of the motor flange 71 is fitted inside a motor holding bore 73 formed in the rack and pinion housing 21. In the case of the first embodiment, the right side end (in
The motor flange 71 is secured to the rack and pinion housing 21 and the belt housing 31 by three bolts 81. Since bolt holes 83 formed on the rack pinion housing 21 and the belt housing 31 have an arc shape as shown in
In the following, the operation of the first embodiment will be described.
When the driver turns the steering wheel 5, the rotational force is transmitted to the steering gear 11 via the upper steering shaft 3 and the lower steering shaft 9. Since the rack and pinion mechanism for converting rotational input into linear movement is provided in the steering gear 11, the rack shaft 23 is moved either to the left or right, so that the steering angle of the wheels is changed by means of the right and left tie rods 15 to achieve steering.
Simultaneously, in the power assist mechanism, the electric motor 35 rotates in the forward or backward direction with a predetermined running torque based on an output of a steering torque sensor (not shown). The rotation is transmitted, while reduced, through the drive pulley 37 and the driven pulley 39 to the ball nut 51 that is spline engaging the driven pulley 39. When the ball nut 51 rotates, a thrust force is exerted on the male thread groove 61 of the rack shaft 23 via the steel balls 65 engaging the female thread groove 63 of the ball nut 51. Thus, steering assist is achieved. In connection with this, in this embodiment, since the interior space of the belt housing 31 is isolated from the ball screw mechanism and the rack shaft 23, and the drive pulley 37 and the driven pulley 39 are supported by the prelubricated deep groove ball bearings 43-46, grease or the like for lubricating the ball screw mechanism, the rack shaft 23 and the bearings 43-46 will not fly to attach to the timing belt 41. Therefore, deterioration of the synthetic rubber material of the belt due to swelling can be prevented.
Next, the process of assembling the steering gear 11 and the process of adjusting the tension of the timing belt 41 in the first embodiment will be described. In the assembly process in the first embodiment, as shown in
Then, the motor flange 71 projects beyond the rack and pinion housing 21, and so the assembly engineer inserts the timing belt 41 into the belt passing space 79, 80 to set it on the drive pulley 37 as shown in
Next, the assembly engineer turns the motor flange 71 fitly relative to the rack and pinion housing 21 and the belt housing 31 while measuring the tension of the timing belt 41. When the value of the tension becomes equal to a prescribed value, the assembly engineer maintains the motor flange 71 at that angular phase and secure the motor flange 71 to the rack and pinion housing 21 and the belt housing 31 with the bolts 81. After that, the assembly engineer assembles the rack shaft 23 and the ball screw housing 33 etc. to finish the assembling of the steering gear 11.
With the above-described structure of the first embodiment, an appropriate tension can be given to the timing belt 41 reliably and easily without using a tensioner or the like. Thus, the number of the constituent parts can be reduced, and smooth steering assist and prevention of damages of the timing belt can be realized.
In the case of the first modification, as shown in
In the following, the operation of the first modification of the first embodiment will be described with reference to FIGS. 15 to 17. In the assembling operation of the steering gear 11 in the first modification, the assembly engineer firstly fits the motor flange 71, on which the electric motor 35, the drive pulley 37 and the timing belt 41 etc. are attached, into the motor holding bore 73 of the rack and pinion housing 21 from the right (from the left, in
After the motor flange is made to project beyond the motor flange 71, as shown in
With the above-described structure of the first modification of the first embodiment, the need for a flange cover and bolts is eliminated and the number of components is further reduced. In addition, by sub-assembling the timing belt 41 to the motor flange 71, reduction of the number of assembling steps is also realized.
Next, a second modification of the first embodiment will be described with reference to FIGS. 18 to 22.
The second modification is substantially the same as the first embodiment, and the same parts are designated by the same reference numerals and redundant descriptions will be omitted. The process of attaching the timing belt after fixing the motor flange 71 to the electric motor 35 is the same; what is different is that bearing support portions 71a at both ends of the drive pulley 37 are made as an integral structure by connecting them with an arm portion 71b extending through the internal space of the timing belt 41.
As shown in
As shown in
As shown in
In the above-described structure, as shown in
As per the above, the process of setting the timing belt 41 on the drive pulley 37 is the same as the first embodiment, but since the bearing support portions 71a at both ends of the drive pulley 37 are integrally composed as a part of the motor flange 71, the number of the constituent parts and the number of the steps of the assembling operation can be reduced. This contributes to cost reduction.
While in the first embodiment, the belt space 79, 80 is provided for allowing a space from the outer periphery of the drive pulley 37, this modification has a more simple structure in which only the arm portion 71b is provided. Therefore, the outer diameter of the motor flange 71 (the bearing support portion 71a) can be made smaller. This contributes to downsizing.
Furthermore, while in the first modification, a space for accommodating the belt is provided in the motor flange 71, such a space is not necessary in this modification. This also contributes to downsizing of the motor flange 71.
As described above, according to the first embodiment of the present invention, it is possible to give an appropriate tension to the belt reliably and easily without using a tensioner or the like. Thus, the number of the constituent parts can be reduced, and smooth steering assist and prevention of damages of the timing belt can be realized.
The outer hull of the power assist mechanism is composed of a belt housing 31 secured to the right side (in
To the lower portion (in
The drive pulley 37 is rotatably supported on the housings 21 and 31 via prelubricated deep groove ball bearings (which will be simply referred to as bearings hereinafter) 45, 46 as closed rolling bearings respectively. As shown in
In the ball screw housing 33, the ball nut 51 is rotatably supported by means of a double row angular ball bearing 53. As shown in
On the rack shaft 23, a male thread groove 61 is formed, while on the ball nut 51, a female thread groove 63 is formed, and a number of steel balls 65 serving as circulating balls are provided between the male thread groove 61 and the female thread groove 63. In addition, a circulation piece (not shown) for circulating the steel balls 65 is attached on the ball nut 51.
On the ball nut 51 side end portion of the driven pulley 39, a female spline 67 is formed, and a male spline 69 engaging the female spline 67 is formed on the ball nut 51. Thus, in the assembled state, the driven pulley 39 and the ball nut 51 rotate together.
In the second embodiment, as tension adjusting means, there is provided a pulley holder 71 for rotatably holding the drive pulley 37 and a holder angle adjusting mechanism 73 for swinging the pulley holder 71. The pulley holder 71 rotatably supports the drive pulley 37 by means of bearings 43 and 44. In addition, a swing shaft 77 held by the rack and pinion housing 21 and the belt housing 31 is inserted through a through hole 75 of the pulley holder 71. A pulley holder 71 is provided with a lever portion 79 projecting from the belt housing 31 side portion thereof (i.e. the upper portion in
The electric motor 35 is secured to the rack and pinion housing 21 with three bolts 101. Since the bolt holes 103 to 105 formed on the rack and pinion housing 21 are long holes (or arcuate holes) as shown in
In the following, the operation of the second embodiment will be described.
When the driver turns the steering wheel 5, the rotational force is transmitted to the steering gear 11 via the upper steering shaft 3 and the lower steering shaft 9. Since the rack and pinion mechanism for converting the rotational input into linear movement is provided in the steering gear 11, the rack shaft 23 is moved either to the left or right, so that the steering angle of the wheels is changed by means of the right and left tie rods 15 to achieve steering.
Simultaneously, in the power assist mechanism, the electric motor 35 rotates in the forward or backward direction with a predetermined running torque based on an output of a steering torque sensor (not shown). The rotation is transmitted, while reduced, through the drive pulley 37 and the driven pulley 39 to the ball nut 51 that spline engages the driven pulley 39. When the ball nut 51 rotates, a thrust force is exerted on the male thread groove 61 of the rack shaft 23 via the steel balls 65 engaging the female thread groove 63 of the ball nut 51. Thus, steering assist is achieved. In connection with this, in the second embodiment, since the interior space of the belt housing 31 is isolated from the ball screw mechanism and the rack shaft 23, and the drive pulley 37 and the driven pulley 39 are supported by the prelubricated deep groove ball bearings 43-46, grease or the like for lubricating the ball screw mechanism, the rack shaft 23 and the bearings 43-46 will not fly to attach to the timing belt 41. Therefore, deterioration of the synthetic rubber material of the belt due to swelling can be prevented.
Next, the process of adjusting the tension of the timing belt 41 in the second embodiment will be described. In the second embodiment, the assembly engineer turns the adjust bolt 87 in the tightening direction with a tool such as a hexagonal wrench to swing the pulley holder 71 in the anticlockwise direction in
After confirming that the tension of the timing belt 41 has become a proper value based on the tightening torque of the adjust bolt 87, the assembly engineer tightens the lock nut 89 to prevent loosening of the adjust bolt 87. After completion of the tension adjustment, the assembly engineer tightens the bolt 101 to secure the electric motor 35 to the rack and pinion housing 21.
Next, the assembling engineer secures the ball screw housing 33 to the belt housing 31 to finish the assembling operation of the steering gear 11. In connection with the above, members such as the rack shaft 23 and the ball nut 51 have been assembled to the ball screw housing 33 in advance in a different assembly line, and when the ball screw housing 33 is connected with the belt housing 31, the female spline 67 of the driven pulley 39 comes into engagement with the male spline 69 of the ball nut 51.
With the above-described structure of the second embodiment, the operation of adjusting the tension of the timing belt 41 can be made accurate and easy. In addition, since the power transmission mechanism and the ball screw mechanism can be assembled and adjusted independently from each other, the assembly process of the steering gear 11 can be made easy.
In this modification, the shaft 36 of the electric motor 35 is connected to the drive pulley 37 via an Oldham coupling 111. As shown in a perspective view presented as
With the above-described structure of the modification of the second embodiment, even if there is a misalignment between the shaft 36 of the electric motor 35 and the drive pulley 37, power transmission is achieved without significant transmission loss by virtue of sliding of the keys 121, 123 relative to the slider 117. Therefore, the assembly engineer need not to swing the electric motor 41 in adjusting the tension of the timing belt nor tightening the bolt 101 after tension adjustment.
Although in the second embodiment the pulley holder is driven by a screw mechanism, it may be driven by other mechanisms such as a cam mechanism. Furthermore, in the modification of the second embodiment, the Oldham coupling may be replaced by an elastic coupling or the like.
As per the above, according to the second embodiment of the present invention, it is possible to give a proper tension to the belt reliably and easily without using a tensioner or the like. Thus, the number of the constituent parts can be reduced, and smooth steering assist and prevention of damages of the timing belt can be realized.
Next, the third embodiment of the present invention will be described.
The outer hull of the power assist mechanism is composed of a belt housing 31 secured to the right side (in
On the lower portion of the belt housing 31, an electric motor 35 is attached. The belt housing 31 accommodates a drive pulley 37 fixed to the shaft 36 of the electric motor 35, a hollow driven pulley 39 connected to a ball nut 51 that will be described later and a timing belt 41 looped around the drive pulley 37 and the driven pulley 39, which constitute a power transmission mechanism. As shown in
In the ball screw housing 33, a ball nut 51 is rotatably supported by means of a double row angular ball bearing 53. Referring to
On the rack shaft 23, a male thread groove 61 is formed, while on the ball nut 51, a female thread groove 63 is formed, and a number of steel balls 65 serving as circulating balls are provided between the male thread groove 61 and the female thread groove 63. In addition, a circulation piece (not shown) for circulating the steel balls 65 is attached on the ball nut 51.
On the ball nut 51 side end portion of the driven pulley 39, a female spline 67 is formed, and a male spline 69 engaging the female spline 67 is formed on the ball nut 51. Thus, in the assembled state, the driven pulley 39 and the ball nut 51 rotate together.
At the end portion of the electric motor 35, a motor flange 71 having a substantially cylindrical shape is secured by bolts. The fitting portion 72 of the motor flange 71 is fitted inside a motor holding bore 73 formed in the rack and pinion housing 21.
The motor flange 71 is secured to the rack and pinion housing 21 and the belt housing 31 by three bolts 81. Since bolt holes 83 formed on the rack and pinion housing 21 and the belt housing 31 have an arc shape as shown in
In the following, the operation of the third embodiment will be described.
When the driver turns the steering wheel 5, the rotational force is transmitted to the steering gear 11 via the upper steering shaft 3 and the lower steering shaft 9. Since the rack and pinion mechanism for converting rotational input into linear movement is provided in the steering gear 11, the rack shaft 23 is moved either to the left or right, so that the steering angle of the wheels is changed by means of the right and left tie rods 15 to achieve steering.
Simultaneously, in the power assist mechanism, the electric motor 35 rotates in the forward or backward direction with a predetermined running torque based on an output of a steering torque sensor (not shown). The rotation is transmitted, while reduced, through the drive pulley 37 and the driven pulley 39 to the ball nut 51 that spline engages the driven pulley 39. When the ball nut 51 rotates, a thrust force is exerted on the male thread groove 61 of the rack shaft 23 via the steel balls 65 engaging the female thread groove 63 of the ball nut 51. Thus, steering assist is achieved. In connection with this, in this embodiment, since the interior space of the belt housing 31 is isolated from the ball screw mechanism and the rack shaft 23, and the drive pulley 37 and the driven pulley 39 are supported by the prelubricated deep groove ball bearings 43-46, grease or the like for lubricating the ball screw mechanism, the rack shaft 23 and the bearings 43-46 will not fly to attach to the timing belt 41. Therefore, deterioration of the synthetic rubber material of the belt due to swelling can be prevented.
Next, the process of adjusting the tension of the timing belt 41 in the third embodiment will be described. In the assembly process of the steering gear 11 in the third embodiment, as shown in
Next, the assembly engineer rotationally drives the drive pulley 37 using a running torque measuring device 95 to revolve the motor flange 71 until the drive resistance (or the running torque) thereof becomes equal to a prescribed value. In the case of the third embodiment, the friction in driving the power transmission mechanism is extremely small and the drive resistance is caused for the most part by the rotation resistance of the bearings 43 to 46. Therefore, if the relationship between the tension of the timing belt 41 and the drive resistance is determined in advance by experiments, it is possible to control the tension of the timing belt 41 quite accurately. In connection with this, the experiments may be performed using a known tension measuring process, such as the natural vibration frequency method or the pressing method. In the drive resistance measuring process, driven pulley 39 may be rotated instead of the drive pulley 37.
After completion of the adjustment of the tension of the timing belt 41, the assembly engineer tightens the bolts 81 with a predetermined tightening torque to secure the motor flange 71 to the rack and pinion housing 21 and the belt housing 31. Next, as shown in
With the above-described structure of the third embodiment, the operation of adjusting the tension of the timing belt 41 can be made accurate and easy. In addition, since the power transmission mechanism and the ball screw mechanism can be assembled and adjusted independently from each other, the assembly process of the steering gear 11 can be made easy.
Although in the above-described third embodiment, a motor flange serving as an eccentric shaft is used as the timing belt tension adjusting means, the tension adjustment may be effected by a different member such as an idler pulley. The belt retaining flanges may be provided on both ends of the driven pulley, or may be provided on one end of the drive pulley and the other end of the driven pulley.
As per the above, in the electric power steering apparatus according the third embodiment of the present invention, the tension of the belt can be estimated based on the drive resistance or the like of the drive pulley or the driven pulley while operating the tension adjusting means in the state in which the ball screw housing has not been secured, and the steering gear can be assembled by, for example, securing the belt housing and the ball screw housing after completion of the tension adjustment. Thus, the operation of adjusting the tension of the belt can be performed accurately and easily.
The outer hull of the power assist mechanism is composed of a belt housing 31 secured to the right side (in
On the lower portion of the belt housing 31, an electric motor 35 is attached. The belt housing 31 accommodates a drive pulley 37 fixed to the shaft 36 of an electric motor 35, a hollow driven pulley 39 connected to a ball nut 51 that will be described later and a timing belt 41 looped around the drive pulley 37 and the driven pulley 39, which constitute a power transmission mechanism. As shown in
In the ball screw housing 33, a ball nut 51 is rotatably supported by means of a double row angular ball bearing 53. Referring to
On the rack shaft 23, a male thread groove 61 is formed, while on the ball nut 51, a female thread groove 63 is formed, and a number of steel balls 65 serving as circulating balls are provided between the male thread groove 61 and the female thread groove 63. In addition, a circulation piece (not shown) for circulating the steel balls 65 is attached on the ball nut 51.
On the ball nut 51 side end portion of the driven pulley 39, a female spline 67 is formed, and a male spline 69 engaging the female spline 67 is formed on the ball nut 51. Thus, in the assembled state, the driven pulley 39 and the ball nut 51 rotate together.
In the belt housing 31, a tension roller 71 that is adapted to be moved back and forth relative to the web surface of the timing belt 41, a roller holder 73 rotatably holding the tension roller 71 and a roller pressing mechanism for pressing the tension roller 71 against the timing belt 41 are provided as tension adjusting means.
The roller holder 73 is rotatably supported on a swing shaft 85 fitly held in support holes 81 formed on the rack and pinion housing 21 and the belt housing 31. The roller holder 73 is a steel press formed part, and it supports a roller shaft 89 that supports the tension roller 71 by means of needle bearings 87 in such a way that the roller shaft 89 is rotatable and slidable in the axial direction within a certain range, as shown in
The roller pressing mechanism 77 is disposed below the roller holder 73 in
In the following, the operation of the fourth embodiment will be described.
When the driver turns the steering wheel 5, the rotational force is transmitted to the steering gear 11 via the upper steering shaft 3 and the lower steering shaft 9. Since the rack and pinion mechanism for converting rotational input into linear movement is provided in the steering gear 11, the rack shaft 23 is moved either to the left or right, so that the steering angle of the wheels is changed by means of the right and left tie rods 15 to achieve steering.
Simultaneously, in the power assist mechanism, the electric motor 35 rotates in the forward or backward direction with a predetermined running torque based on an output of a steering torque sensor (not shown). The rotation is transmitted, while reduced, through the drive pulley 37 and the driven pulley 39 to the ball nut 51 that spline engages the driven pulley 39. When the ball nut 51 rotates, a thrust force is exerted on the male thread groove 61 of the rack shaft 23 via the steel balls 65 engaging the female thread groove 63 of the ball nut 51. Thus, steering assist is achieved.
In connection with this, in the fourth embodiment, since the belt retaining flanges 91 are provided at both end portions of the tension roller 71 for giving tension to the timing belt 41, and the tension roller 71 is movable in the axial direction, problems such as dislocation of the timing belt 41 from the tension roller 71 or meandering of the timing belt 41 due to displacement of the tension roller 71 will not occur.
In addition, in the fourth embodiment, since the interior space of the belt housing 31 is isolated from the ball screw mechanism and the rack shaft 23, and the drive pulley 37 and the driven pulley 39 are supported by the prelubricated deep groove ball bearings 43-46, grease or the like for lubricating the ball screw mechanism, the rack shaft 23 and the bearings 43-46 will not fly to attach to the timing belt 41. Therefore, deterioration of the synthetic rubber material of the belt due to swelling can be prevented.
Next, the process of adjusting the tension of the timing belt 41 in the fourth embodiment will be described. In this fourth embodiment, the assembly engineer turns the adjust bolt 99 in the tightening direction with a tool such as a hexagonal wrench to swing the roller holder 73 in the clockwise direction in
In this process, as shown in
After confirming that the tension of the timing belt has become a proper value based on the tightening torque of the adjust bolt 99, the assembly engineer tightens the lock nut 101 to prevent loosening of the adjust bolt 99. As shown in
Although in the fourth embodiment the structure in which a roller holder supported in a swingable manner is pressed by a screw mechanism is adopted as the timing belt tension adjusting means, a linearly movable roller holder or a roller holder driven by a cam mechanism may be used.
According to the electric power steering apparatus according to the fourth embodiment of the present invention, in the rack assist type electric power steering apparatus including a ball screw mechanism composed of a ball nut having a female thread groove formed on its inner circumferential surface, a rack shaft disposed at the central axis of the ball nut and having a male thread groove that is opposed to said female thread groove, formed on its outer circumferential surface and a plurality of circulating balls disposed between said female thread groove and said male thread groove, an electric motor disposed non-coaxially with said rack shaft for driving said ball nut mechanism, power transmission means composed of a drive pulley rotating integrally with the shaft of the electric motor, a driven pulley rotating integrally with said ball nut and a belt looped around the drive pulley and the driven pulley, a tension adjusting means for adjusting the tension of said belt, and a steering gear case for accommodating said ball screw mechanism and said power transmission means, said tension adjusting means has a tension roller that is rotatably supported by a roller holder disposed outside said belt and adapted to be moved back and forth relative to a web surface of said belt together with the roller holder, and when the tension roller is pressed against the outer surface of the belt, the wrapping angle of the belt on both the pulleys is increased, so that the shear stress of the belt at the pulley having the relatively smaller diameter is reduced. Thus, the durability of the belt is enhanced.
Although the description of the specific embodiments has been made, modes of the present invention are not limited to the above-described embodiments. For example, although in the above two embodiments, the present invention is applied to arrangements in which a timing belt is used in the power transmission mechanism, the present invention may also be applied to arrangements in which a V-belt or a flat belt is used. Furthermore, the closed rolling bearing may be a bearing other than the prelubricated deep groove ball bearing. Besides the above, the overall structure of the steering gear and the specific structure of the power assist mechanism and the ball screw mechanism are not limited to the illustrative examples in the above-described embodiments, but they can be modified fitly in accordance with requirements of design or specifications without departing from the spirit and scope of the present invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2002-242520 | Aug 2002 | JP | national |
| 2002-268845 | Sep 2002 | JP | national |
| 2002-259986 | Sep 2002 | JP | national |
| 2002-251286 | Aug 2002 | JP | national |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/JP03/10555 | Aug 2004 | US |
| Child | 11053556 | Feb 2005 | US |
