Patent Application
20150111674
The invention relates to a CVT clutch comprising an inertia member disposed between a back plate and a moveable sheave, the inertia member radially moveable upon a radially extending surface upon rotation of the moveable sheave.
A typical CVT transmission is made up of a split sheave primary drive clutch connected to the output of the vehicle engine (often the crankshaft) and split sheave secondary driven clutch connected (often through additional drive train linkages) to the vehicle axle. An endless, flexible, generally V-shaped drive belt is disposed about the clutches. Each of the clutches has a pair of complementary sheaves, one of the sheaves being movable with respect to the other. The effective gear ratio of the transmission is determined by the positions of the movable sheaves in each of the clutches.
The primary drive clutch has its sheaves normally biased apart (e.g., by a compression coil spring), so that when the engine is at idle speeds, the drive belt does not effectively engage the sheaves, thereby conveying essentially no driving force to the secondary driven clutch. The secondary driven clutch has its sheaves normally biased together (e.g., by a compression or torsion spring working in combination with a helix-type cam, as described below, so that when the engine is at idle speeds the drive belt rides near the outer perimeter of the driven clutch sheaves.
The axial spacing of the sheaves in the primary drive clutch usually is controlled by centrifugal flyweights. Centrifugal flyweights are operably connected to the engine shaft so that they rotate along with the engine shaft. As the engine shaft rotates faster (in response to increased engine speed) the flyweights also rotate faster and pivot outwardly, urging the movable sheave toward the stationary sheave. The more radially outwardly the flyweights move the more the moveable sheave is axially moved toward the stationary sheave. This pinches the drive belt, causing the belt to begin rotating with the drive clutch, the belt in turn causing the driven clutch to begin to rotate.
Further movement of the device clutch's movable sheave toward the stationary sheave forces the belt to climb radially outward on the drive clutch sheaves, increasing the effective diameter of the drive belt path around the drive clutch. Thus, the spacing of the sheaves in the drive clutch changes based primarily on engine speed. The drive clutch therefore can be said to be speed sensitive, and is also called the speed governor.
As the sheaves of the drive clutch pinch the drive belt and force the belt to move radially outward on the drive clutch sheaves, the belt is pulled radially inward between the sheaves of the driven clutch, decreasing the effective diameter of the drive belt path around the driven clutch. This movement of the belt on the drive and driven clutches smoothly changes the effective gear ratio of the transmission in variable increments. Tuning the engagement speed is accomplished by a combination of the pre-load of the compression spring and the mass. The device provides a smooth transition for the vehicle from a full stop. The disadvantage is the extra cost and added on mass.
Representative of the art is U.S. Pat. No. 5,460,575 which discloses a drive clutch assembly having a fixed sheave and a movable sheave rotatable with the drive shaft of an engine comprising a variable rate biasing or resistance system for urging a movable sheave toward a retracted position, the biasing system initially applies a first predetermined resistance to the movable sheave as it moves toward the fixed sheave and applies a second predetermined resistance to the movable sheave when the movable sheave reaches a predetermined axial position.
What is needed is a CVT clutch comprising an inertia member disposed between a back plate and a moveable sheave, the inertia member radially moveable upon a radially extending surface upon rotation of the moveable sheave. The present invention meets this need.
An aspect of the invention is to provide a CVT clutch comprising an inertia member disposed between a back plate and a moveable sheave, the inertia member radially moveable upon a radially extending surface upon rotation of the moveable sheave.
Other aspects of the invention will be pointed out or made obvious by the following description of the invention and the accompanying drawings.
The invention comprises a CVT drive system comprising a moveable sheave axially moveable along a first shaft and having a radially extending surface, a fixed sheave fixed to the first shaft, the fixed sheave cooperatively disposed with the moveable sheave to engage a belt therebetween, the first shaft engagable with an engine output, a back plate attached to the first shaft and having a radial surface, the back plate engaged with the moveable sheave for a locked rotation while allowing a relative axial movement, an inertia member radially moveable upon the radially extending surface and the radial surface upon rotation of the moveable sheave, the inertia member is temporarily disengagable from the radial surface and from the radially extending surface, a first spring resisting axial movement of the moveable sheave toward the fixed sheave along the first shaft, and a sleeve member disposed between the moveable sheave and the fixed sheave, the sleeve member rotatable with the belt.
The accompanying drawings, which are incorporated in and form a part of the specification, illustrate preferred embodiments of the present invention, and together with a description, serve to explain the principles of the invention.
Sheave 50 has a sliding engagement with bush 40 and shaft 30. Step 41 at an outside diameter of bush 40 forms a spring seat. Spring 70 is disposed between spring seat 41 and spring cup 80. Spring 70 resists movement of moveable sheave 50 toward sheave 100. Sleeve 60 engages the bearing 90 outer raceway 91 to support the belt when the belt (not shown) is in the radially inward position. Bearing 90 inner raceway 92 engages and rotates with shaft 30. Sleeve 60 covers spring 70 to prevent engagement of the belt with spring 70. Further, spring cup 80 contacts and rotates with the inner raceway 92 of bearing 90. Spring cup 80 together with spring seat 41 locate spring 70 within the mechanism. Sheave 100 is fixedly attached to an engine output shaft (not shown) by a splined joint.
The system may use a plurality of inertia members 20. The instant embodiment comprises six members 20 by way of example and not of limitation. Each member 20 comprises a mass. The mass of each member determines the radial force each develops as a function of the rotational speed of the clutch. The amount of mass used in each member is adjustable by adding an insert 21 to a member or members, see
For a given mass (m) and number of members 20 one may determine the total force which will be exerted against the force of spring 70 as the clutch rotates. This in part determines the operational characteristics of the system such as at which speeds radially outward movement of the members 20 takes place overcoming the spring force and thereby causing axial movement of movable sheave 50 toward sheave 100 against the spring force 70. In other words: F=mrω2, the total centrifugal force (F), which acts in radial outward direction is balanced by the reaction forces from both the back place 10 and from the sheave 50.
Both back plate 10 and sheave 50 have surfaces (51,11) which are inclined to a normal extending radially from the shaft. The reaction force between each member 20 and the moveable sheave 50 has a component that is projected in the axial direction along the axis of rotation A-A. The axial force exerted on the moveable sheave 50 is cumulative depending upon the number of members 20 used in the clutch and the profile of the surface 51 and surface 11, see
Members 20 are disposed in a radially inward position (small radius from axis of rotation A-A) during low rotational speed conditions. This represents the position of greatest separation between the movable sheave 50 and stationary sheave 100. As the rotational speed increases the members move radially outward and moveable sheave 50 moves toward sheave 100.
Sheave collar 260 is attached to sheave 270. Sheave collar 260 comprises one or more helically shaped slots 261 which partially wrap about collar 260. Each slot 261 extends in an axial direction parallel to axis A-A. Each guide member 300 either rollingly or slidingly engages a slot 261. Engagement of the guide member 300 with a slot 261 prevents rotation of sheave 270 with respect to sheave 310 during operation, although the helical form of slot 261 allows some small amount of relative rotational movement.
Guide member 300 provides at least two functions. First, it provides for the capability to transfer the belt “pull” force from sheaves 270 and 310 to the output shaft 290. Each member 300 also serves as the reaction point to load sensing feedback from slot 261 in the moveable sheave 270. Slot 261 is also called the torque reactive ramp, which converts the driven torque into the axial force which moves the moveable sheave 270 in response to a torque change.
Guide 300 further comprises an outer roller portion 301 which facilitates movement of the guide 300 within slot 261. Nut 320 holds the driven clutch assembly together.
In the disclosed embodiment surface 11 has a planar profile and surface 51 has an arcuate profile. Each profile regulates the rate and radial extent of the movement of each member 20 as it moves radially inward and outward during engine operation. Each surface profile may be adjusted as needed to accommodate the desired rotational characteristic of the clutch.
For example, the profile of surface 11 and surface 51 will affect the radially inward and outward movement of each member 20 as the clutch speed varies. Namely, depending upon the profile each member may have to “climb” up the surface 51 and surface 11 as it moves radially outward, which in turn will affect the rate at which sheave 50 moves toward sheave 100, or, will affect the speed at which each member 20 will be disposed at a desired radial position, which will correspond to a given gear ratio. One skilled in the art can appreciate that selection of a surface 11 and surface 51 profile can be used to affect clutch behavior over a desired speed range.
By way of example and not of limitation, the profile of surface 51 can be arcuate, parabolic, planar, a circular section and so on. In the case of a planar section the angle at which the plane is disposed to a normal radially extending from the shaft axis A-A can be used to affect the rate or speed at which the members 20 will move radially outward during operation. The profile of surface 11 can be arcuate, parabolic, planar, a circular section and so on. In the case of a planar section the angle at which the plane is disposed to a normal radially extending from the shaft axis A-A can be used to affect the rate or speed at which the members will move radially outward during operation.
In the open position each member 20 is disposed in a more radially inward position between back plate 10 and sheave 50. In the radially inward position a space (S) exists such that member 20 is not fixedly captured between back plate 10 and sheave 50 and surface 53 because each member 20 does not simultaneously contact surface 11, surface 51 and surface 53. Members 20 do not necessarily roll along the surface 51 or surface 11. Instead, a member 20 may also slide against surface 51 and surface 11, or a member may slide against one surface and roll across the other. In order to prevent a flat spot developing on the member 20 due to friction or abrasion, a relief shoulder 12 prevents pinching of the member by surface 51 and surface 11.
In the fully open sheave condition the spring 70 force is prevented from being applied to each member 20 by sheave 50 and sheave 100 by a relief shoulder 12, as shown in
Two methods are available to achieve the fully closed position for the sheaves: displacement control and force control.
In the alternative by extending surface 51 and back plate surface 11 radially outward, thereby preventing a member 20 from contacting flat surface 52, sheave 50 axially moves until it contacts stationary sheave 100. This is the limit of axial movement of sheave 50 and is called displacement control. Displacement control has an advantage over the force control since it allows one to extend the range of the speed ratio change, which can improve the top end speed of a vehicle using the inventive system.
In operation, instead of using a known centrifugal clutch which is typically placed at the driven clutch assembly position to engage and dis-engage the engine at the idle speed, in the instant clutch the CVT belt is used as the clutching mechanism. Advantages of using a belt clutch include cost savings and improved fuel economy.
In particular, the belt used in the inventive clutch is typically shorter than a belt for a known centrifugal clutch system. Use of a shorter belt forces the driven clutch open slightly, that is, sheave 270 and sheave 310 are forced slightly apart. An initial tension on the belt is developed by spring 210 in
During engine idle the CVT belt 400 is resting on the sleeve 60 and driver bearing 90, see
The initial gap (“gap”) at the driven clutch, as shown in
Spring 70 at the driver clutch is used to control the engine belt engagement speed. The greater the compressive spring rate for spring 70, the higher the engine speed required to overcome the spring force and thereby cause sheave 50 to move toward sheave 100, and thereby engage the belt.
Referring to
At engine idle the belt rests against sleeve 60 while spring 70 rotates together with the driver sheave 50. Given gap (G) the belt is not rotating. As the engine rotational speed increases centrifugal force is developed for each member 20 according to the mass of each member. The centrifugal force urges each member 20 radially outward along surface 11 and surface 51, which force has a component oriented axially along shaft 30. This urges moveable sheave 50 closer to the belt and to sheave 100. As the engine speed exceeds the engagement speed, moveable sheave 50 and sheave 100 engage, or “pinch”, the belt. The rotary motion and torque of the engine are then transmitted by the belt from the driver clutch to the driven clutch. Since the belt is pre-tensioned by the engagement of the driven mechanism there is no jerk motion when the driver sheave engages the belt. The engine engagement speed can be tuned by changing the compressive spring rate of spring 70, or by changing the magnitude of the mass of each member 20.
The inventive system achieves smooth engagement transition on engine acceleration. Faster acceleration can also be achieved because the belt slips much less than a prior art centrifugal clutch after the engagement of the belt. The engagement characteristic can also be established based upon the mass and number of each roller. It is also a function of the profile of the radially extending surface and surface 11. For example, a steeper profile for surface 11 and surface 51 will require greater centrifugal force to move the members radially outward, and vice versa.
During a downshift, i.e., the CVT drive shifts from the over drive condition (low speed ratio) to the under drive condition (high speed ratio), it is preferable that the engine remains constantly engaged with the vehicle driveline to take advantage of the engine braking effect. Engine braking is achieved in the inventive system by selecting a proper compression spring 70 pre-load in the driver clutch. In the inventive system an exemplary spring pre-load is 100N. For example, if the pre-load of spring 70 is too high, the driver clutch will open prematurely as the engine speed slows down. If both the driven clutch and driver clutch open simultaneously the belt can lose engagement with the driver and driven clutches and thereby lose tension. This will allow the belt to slip. This in turn can dis-engage the engine losing engine braking which may lead to a runaway situation. On the other hand, if the pre-load of spring 70 is properly selected to maintain the gap (G) during engine idle, the driver clutch will not open prematurely as the engine speed drops from the drive condition. Instead, the driven clutch sheaves will not prematurely move apart thereby holding the belt engaged in a radially outward position. The belt can then press radially inward to force open the driver clutch sheaves during a downshift. Hence, belt tension is maintained during a downshift to allow the CVT to fully utilize engine braking.
A driving cycle from India is used for the test. The test is different from that used in other countries because initial vehicle cost and fuel economy are the highest priorities, and the engine size for the majority of vehicles is under 125 cc. The test comprises the following parameters.
The fuel economy test was conducted on a chassis dynamometer. A scooter equipped with a prior art CVT clutch was tested, namely, prior art system “B”. The same scooter was then tested using the inventive CVT clutch as described in this specification as inventive system “A”. The same engine and fuel were used for both tests.
At all tested speeds the constant speed fuel economy of the inventive CVT system “A” is significantly greater than the prior art centrifugal clutch system “B”. The fuel economy improvement ranges from 11% at the upper and lower speed points up to 32% for 45 km/hr.
On the other hand, the inventive system achieves a much lower engagement engine speed in the range of approximately 2000 RPM, see curve “A” of
Although a form of the invention has been described herein, it will be obvious to those skilled in the art that variations may be made in the construction and relation of parts without departing from the spirit and scope of the invention described herein.
