This application relates to preloaded bearings and provides for a supercharger housing with preloaded rotor bearings.
Twin screw and Roots superchargers are subject to chatter and other vibration errors as the rotors spin in the housing. The vibrations can be caused by tolerance stack-ups, they can be temperature dependent as parts expand and contract, they can be driven by shaft instabilities, whirl, internal bearing slip at contact surfaces and rattle within assembly clearances. Vibrations can be along the rotor axis or perpendicular to it. When a bearing is mounted to the rotor shafts, the bearings can squeal in response to the vibrations or in response to temperature-sensitive tolerances.
Clearances in the rotor bore and bearing assemblies are a source of vibration. The clearances allow the rotor to move in the axial and radial direction and vibrate. The movement and vibration can result in reduced performance and objectionable noise. Also, the rotors can contact the rotor bore, resulting in coating wear and damage to both the rotors and the rotor bore.
The method and devices disclosed herein overcome the above disadvantages and improve the art by way of a supercharger housing adapted to preload a bearing.
A supercharger comprises a housing, a gear box, and a shaft. The shaft comprises a first end and a second end, wherein the first end is located closer to the gear box than the second end. The supercharger comprises a shaft bore comprising a base wall, wherein the second end of the shaft is located in the shaft bore. The supercharger comprises a rotor bore in the housing and a rotor located on the shaft in the rotor bore. The rotor comprises an axis. The supercharger comprises a bearing surrounding the shaft and located closer to the second end of the shaft than the first end of the shaft, wherein the bearing comprises an outer ring abutting the shaft bore and an inner ring abutting the shaft. The supercharger comprises a biasing device abutting the bearing, wherein the biasing device moves the rotor along the axis.
A supercharger comprises a housing, a first shaft, a second shaft, a rotor bore in the housing, a first rotor located on the first shaft in the rotor bore, a second rotor located on the second shaft in the rotor bore, and a first helical gear connected to the first shaft. The first helical gear comprises a plurality of helical teeth. The supercharger comprises a second helical gear connected to the second shaft. The second helical gear comprising a plurality of helical teeth.
A method for assembling a supercharger comprises fixing a rotor to a shaft, wherein the shaft comprises an axis. The method comprises installing a biasing device into a shaft bore, wherein the biasing device abuts a base wall. The method comprises installing the rotor into a rotor bore, installing the shaft into the shaft bore, installing a bearing into the shaft bore, installing the bearing onto the shaft, and applying a force against the bearing with the biasing device, thereby moving the rotor along the axis.
Additional objects and advantages will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the disclosure. The objects and advantages will also be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the claimed invention.
Reference will now be made in detail to the examples, which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. Directional references such as “left” and “right” are for ease of reference to the figures.
When a bearing is mounted to the rotor shafts of a supercharger, the bearings can squeal in response to vibrations, temperature-sensitive tolerances, or tolerance stack-up. Another cause for noise is boost load, where the variation in pressure of air moving through the housing as the rotors turn causes changes in magnitude and orientation of load against the rotors. This may also include unloaded conditions, in which the changes in magnitude and direction of rotor load caused by the boost load can also similarly vary the bearing load. The load can be along the rotor axis or perpendicular to it. Preloading the bearing helps solve this problem by minimizing operating bearing clearances, thus, reducing unwanted noise and vibration.
Clearances in the bearings and gear box of a supercharger can cause the rotor to move during operation. This movement can be in the form of vibration, axial displacement, radial movement, or a combination of the all these movements. As a result of this movement, the rotor can rub or hit against the housing, wearing off coatings and damaging the rotor. This can result in decreased performance because the damaged rotor loses volumetric efficiency. It can also lose symmetry, becoming less stable when rotating.
This movement can also cause undesirable noise, vibration, and harshness (NVH), especially when the supercharger operates in cold environments, for example, at temperatures near −40 degrees centigrade. This noise is sometimes called “hoot noise” or “squeal.”
The movement can also cause undesirable noise, vibration, and harshness at high temperatures. The housing and parts of a supercharger can reach temperatures exceeding 200 degrees centigrade during operation. Thus, a supercharger must be designed to operate within a wide range of temperatures, for example, within a range of −40 degrees centigrade to 200 degrees centigrade.
The change in temperature causes the rotor to move because the rotor and other parts expand when the temperature increases and contract when the temperature decreases. Parts are often made from different materials, including aluminum and steel. Because the parts are made from different materials, they expand and contract at different rates.
When exposed to cold temperatures, the housing can contract, resulting in less clearance between the rotor and the housing. This increases the risk that the rotor contacts the housing.
The rotor can also move due to mechanical strain on the rotor and bearings. These strains are caused by loads experienced during operation, for example, loads caused by boost pressure and thrust from helical gears.
When the bearing is preloaded in line with the load placed on the rotors via the boosting process, as shown in
Gears 180, 181 can be helical gears. Helical gear 180 has the same lead as helical gear 181. Lead is the axial advance of a helix for one complete turn. Lead can be calculated using equation (1), where
pz=axial lead
z=number of teeth
mn=normal module
β=helix angle
Helical gears 180, 181 can also rotate at the same rate of speed as rotors 130, 131 even when rotors 130, 131 move axially due to changes in axial load, for example, due to changes in thrust force and boost pressure.
In
Helical gears 680, 681 can rotate at the same rate as the rotors 630, 631. One feature of an axial-inlet, radial-outlet supercharger is that the rotors 630, 631 have a helical twist along axes A, B. Rotors 630, 631 have a plurality of lobes, for example, lobes 632, 633. Lobes 632, 633 are helices. Lobes 632, 633 have helix angles with respect to axes A, B. All the lobes have the same magnitude helix angle, but the helix angle β3 of lobes 632 on rotor 630 are opposite in direction from the helix angle β4 of lobes 633 on rotor δ31. For example, helix angles β4 and β3 are equal in magnitude, but β4 is negative and β3 is positive when rotor 630 is right-handed and rotor 631 is left-handed. They also have the same magnitude lead, which can be calculated using equation (1).
Helical gears 680, 681 also have a twist along axes A, B. Helical gears 680, 681 have teeth, for example, teeth 682, 683. Teeth 682, 683 are helices. Like lobes 632, 633, teeth 682, 683 have a helix angle with respect to axes A, B. All the teeth have the same magnitude helix angle, but the helix angle β1 of teeth 682 on helical gear 680 are opposite in direction from the helix angle β2 of teeth 683 on helical gear 681. For example, helix angles β2 and β1 are equal in magnitude, but β2 is negative and β1 is positive when helical gear 680 is right-handed and helical gear 681 is left-handed. The helix angles β2, β1 of teeth 682, 683 on helical gears 680, 681 need not be of the same magnitude as the helix angles β4, β3 of lobes 632, 633 on rotors 630, 631. All the teeth 682, 683, however, have the same lead magnitude as the lead magnitudes of lobes 632, 633.
Gears 680, 681 can be called timing gears. The configuration of the rotor assembly 600 maintains the timing of the rotating rotor group independent of the axial movement of rotor shafts 640, 641. Both gears 680, 681 and rotors 630, 631 twist at the same rate of angular displacement. When gears 680, 681 are synchronized with rotors 630, 631, gears 680, 681 rotate rotor shafts 640, 641 at the same rate as rotors 630, 631, even as the rotor shafts 640, 641 move axially (such as due to bearing internal clearances). In addition, any thermal growth such as axial growth along rotor shafts 640, 641 can occur at the same rate. In this regard, the clearances (gap or channel) between the rotors 630, 631 can be maintained without compromising the rotor coating or reducing efficiency.
The axial movement of shaft 640 can cause helical gear 680 to rotate helical gear 681. And the axial movement of the shaft 641 can cause helical gear 681 to rotate helical gear 680. In the arrangement shown in
When helical gears 180, 181 are used instead of conventional spur gears, the timing of the rotation of rotors 130, 131 remains independent of the axial movement. Spur gears have a helix angle equal to zero. The teeth are not helices, but instead, the teeth in a spur gear are parallel to the shafts axes, for example, axes A, B in
As discussed above using helical gears instead of spur gears also helps better maintain the clearance between the rotors and the housing, improving efficiency and preventing damage. Using helical gears also reduces the noise that often accompanies spur gears.
In one test condition, it was shown that a conventional rotor arrangement, not preloaded by a bearing, can move as much as 0.066 mm when operating at 120 degrees Centigrade and as much as 0.100 mm at 150 degrees Centigrade. Adding a preload of 50 N to bearings 160, 161 in the direction of boost (axial load L1 direction) reduced the axial displacement to a total displacement of 0.013 mm at 120 degrees Centigrade and 0.008 mm at 150 degrees Centigrade. Other results are possible depending upon radial internal clearances of the bearing and boost forces.
The preload can be greater or less than 50 N. One can select the amount of the preload to fit needs of the supercharger. For example, a rotor might experience loads of 75 N during operation. Thus, a preload of more than 75 N can be used to keep the rotor biased toward bearings 160, 161 thereby reducing axial displacement and keeping the rotor positioned at its original location.
The preload can depend on the bearing's dynamic load rating. The International Organization for Standardization (ISO) and bearing manufactures publish dynamic load ratings for bearings. The capacity can be defined as a rating. Having too great of a preload can reduce the lifespan of the bearing. One can select a preload high enough to prevent a zero load condition from occurring but low enough to avoid undesirably reducing the lifespan of the bearing. For example, the preload can be less than 2% of the dynamic load rating. The preload can be greater than 0.5% and less than 2% of the dynamic load rating. Thus, for a large bearing, the preload might exceed 50 N, but still be less than 2% of the dynamic load rating.
The preload can be applied by biasing devices 170, 171, as shown in
Biasing devices 170, 171 can be compression springs, such as wave springs, coils springs, leaf springs, Belleville springs, or disc springs, or other biasing devices. Biasing devices 170, 171 can abut base walls 125, 126 and bearings 160, 161 as shown.
When installed in an axial-inlet, radial outlet supercharger housing 120, the flow pattern through the housing impacts the size of the bearings 160, 161 and the preload of biasing devices 170, 171. For example, a radial-inlet, radial-outlet supercharger can accommodate larger loads on the rotors, and larger bearings in the base walls. The larger design can use larger springs. But, an axial-inlet, radial-outlet supercharger must use smaller bearings to avoid restricting the size of the axial inlet. The change in size of the bearings is not straightforward to implement. The smaller bearings spin faster, but give up load capacity. The biasing devices must be selected for the smaller size, as by reducing the preload. And, the angles of rotors 130, 131 are adjusted, which impacts the helix angles of helix gears 680, 681.
Shaft 140 is attached to bearing 160 and rotor 130. Thus, when biasing device 170 pushes against bearing 160, it pulls shaft 140 and rotor 130 in the direction of L1 along axis A. Likewise, biasing device 171 pulls shaft 141 and rotor 131 along axis B.
The first end 143 of shaft 140 is located in gear box 150. Shaft 141 can be surrounded by second bearing 158 near first end 143. When shaft 140 is pulled in the direction of L1, it moves in the direction of L1 along axis A. This locks shaft 140 and rotor 130 in place, eliminating play allowed by clearances in gear box 150 and second bearing 158. Second bearing 158 can be fixed to gear box housing 151 via an interference fit. This prevents the outer surface 157 of second bearing 158 from moving in the axial direction, but internal bearing components, for example, rollers and inner races, have clearances that allow play.
Bearing 160 can be slip-fit into shaft bore 122. This allows biasing device 170 to push bearing 160, shaft 140, and rotor 130 away from second bearing 158 along axis A.
Conventional superchargers use needle bearings. In the present disclosure, bearings 161, 162 can be deep groove ball bearings. Using ball bearings instead of needle bearings can reduce the axial length and cost of the supercharger. Ball bearings can be less prone to the high motion and noise that accompanies needle bearings.
One can close shaft bores 122, 123 with cover plate 127 after installing the bearings 160, 161 and biasing devices 170, 171 in shaft bores 122, 123. Cover plate 127 can be attached by welding, bolting, screwing, or other fastening methods.
Biasing devices 370, 371 can be installed by placing them in shaft bores 322, 323 from the rotor bore 321. One can first place biasing devices 370, 371 in shaft bores 322, 323, then place bearings 360, 361 in shaft bores 322, 323, and then place shafts 340, 341 in bearings 360, 361, which are already in shaft bores 322, 323. Or one can first place biasing devices 370, 371 and bearings 360, 361 on shafts 340, 341, then place shafts 340, 341 (with bearings 360, 361 and biasing devices 370, 371 surrounding shafts 340, 341) into shaft bores 322, 323. Or one can first place biasing devices 370, 371 into shaft bores 322, 323, then place shafts 340, 341 (with bearings 360, 361 attached to shafts 340, 341) into shaft bores 322, 323. None of these methods requires attaching a separate cover plate over shaft bores 322, 323. Base walls 325, 326 can be built into shaft bores 322, 323 with back wall 327 of shaft bores 323, 324 being an integral part of housing 320.
Bearing 460 has an outer ring 468 and an inner ring 466. Outer ring 468 can be slip-fit into shaft bore 422. Slip-fitting outer ring 468 allows bearing 460 to more easily move in the axial direction along axis A. Shaft 440 can be press-fit into inner ring 466. With outer ring 468 free to move in the axial direction and inner ring 466 attached to shaft 440, bearing 460 pulls on shaft 440 when preloaded with biasing device 470, locking shaft 440 in place.
Or biasing device 470 can abut outer ring 468, but not inner ring 466. In this arrangement, biasing device 470 pushes against outer ring 468. Outer ring 468 can thereby pull on inner ring 466 via balls 467.
The magnitude of the spring preload of the biasing device is set based on the bearing sizes, application duty cycle, rotor geometry and gear geometry in gear box 150. An ideal spring preload is greater than the sum of the opposing axial loads from the rotor operation to prevent the rotor shaft from traversing the axial internal clearance of the fixed end ball bearing. This better maintains the rotor gaps and prevents excessive coating wear. The spring preload can be in-line (in the same direction) as axial loads, such as boost loads, or the spring preload can be opposing the axial load.
The arrangements above can improve supercharger performance by reducing axial movement of the rotor during operation. Eliminating movement due to clearances in the gear box and bearings improves the stability of the rotors during operation.
Other implementations will be apparent to those skilled in the art from consideration of the specification and practice of the examples disclosed herein. It is intended that the specification and examples be considered as exemplary only, with the true scope of the invention being indicated by the following claims.
Filing Document | Filing Date | Country | Kind |
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PCT/US16/15095 | 1/27/2016 | WO | 00 |
Number | Date | Country | |
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62133829 | Mar 2015 | US | |
62174286 | Jun 2015 | US | |
62174287 | Jun 2015 | US |