Speed limiting accessory drive
FIELD OF THE INVENTION 
     The present invention relates to engines in vehicles that employ belt and pulley systems to drive some of the vehicle accessories and more particularly to pulley systems that limit the driven speed of the accessories at high engine speeds. 
     This application is related to a co-pending patent application titled SPEED LIMITING ACCESSORY DRIVE AND CRANKSHAFT DAMPER, filed herewith, and incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     Conventional engines in vehicles not only provide power for the drivetrain but also provide power for accessories. Such accessories may include an air conditioning compressor, a water pump, a power steering pump, an alternator, etc. Generally, these accessories are driven by the engine via driven pulleys coupled to a driving pulley by a belt, with the driving pulley driven by the engine crankshaft. Thus, the accessories are driven at some predetermined ratio of the engine crankshaft speed, with the driving speed changing when the engine speed changes. 
     In today&#39;s vehicles, drivers expect that the accessories will work for all engine operating conditions. Thus, at engine idle, when the engine is generally running its slowest, and therefore the accessories are driven at their slowest speeds, the ratio of accessory driven speed to crankshaft speed must be sufficient to completely power these accessories (i.e., have adequate capacity). However, with this capacity designed-in for low engine speed conditions, when the engine is running at very high speeds (RPMs) the driving speed may be too high and overpower some accessories, creating the possibility for excessive wear of the accessories and additional noise and vibrations. Currently, for a given set of accessories, this situation requires a trade-off, then, between the minimum idle speed allowed for an engine and the maximum speed at which the engine can operate depending upon the ratio of the pulleys. 
     With a fixed pulley system and accessory size, possible solutions to this predicament are to either limit the minimum RPMs for engine idle conditions, thereby allowing for adequate capacity for the accessories, or to limit the upper speed (RPM) range, limiting the potential for overpowering; neither one a very satisfactory solution. The first hurts fuel economy at idle and the second would limit the engine power. 
     Accordingly, it is desirable to provide an accessory pulley system that will allow the accessories to be sized to handle the accessory load (adequate capacity) at low idle speeds, to increase fuel economy, while not over-driving the accessories at high engine speeds and minimizing the cost and complications needed in the pulley system to accomplish this. Furthermore, it is desirable that this system will operate automatically, without the need for external inputs, in order to minimize the complexity of the system. 
     SUMMARY OF THE INVENTION 
     In its embodiments, the present invention contemplates a speed limiting accessory drive, adapted to be rotationally driven by a crankshaft of an engine and drive engine accessories, which limits the maximum speed at which the accessories are driven. The speed limiting accessory drive includes a hub member adapted to be rotationally fixed to the crankshaft, and a housing mounted about and rotatable relative to the hub, the housing including a cavity. A pulley member is mounted on and rotationally fixed relative to the housing. A wet clutch assembly, mounted radially outward within the housing cavity, includes a plurality of friction discs and a plurality of separator plates interleaved with the friction discs, with the friction discs being rotationally fixed relative to the hub and the separator plates being rotationally fixed relative to the housing. A front cover is mounted to the housing for sealing the cavity in the housing, Biasing means are located in the cavity and rotationally fixed relative to the housing, for exerting a first force on the clutch assembly to press the separator plates into contact with the friction discs. Fluid fills the cavity in the housing, whereby the pulley member will be driven at crankshaft speed for low crankshaft rotational speeds, and the fluid will exert a second force on the biasing means opposite the first force to allow clutch slip at a maximum pulley rotational speed for high crankshaft rotational speeds. Temperature compensating means create a third force on the biasing means in the direction of the first force as the temperature of the biasing means rises. 
     Accordingly, an object of the present invention is to provide a speed limiting accessory drive for use in a pulley system of an engine, which limits the maximum speed at which the pulley system drives the accessories, enabling increased drive ratios for the pulley system at low engine speeds over conventional pulley systems. 
     An advantage of the present invention is that the engine can be made to idle at lower speeds, while still providing adequate accessory capacity at these speeds, by employing increased drive ratios for the pulley system, while still not over-driving the accessories at high engine speeds, thereby increasing the fuel economy of the vehicle. 
     A further advantage of the present invention is that the maximum speed of the pulley system is automatically limited without the need for operator or electronic inputs to the system. 
     An additional advantage of the present invention is that the accessories are driven slower at higher engine speeds, thereby increasing operating life of and reducing noise and vibrations produced by the accessories at these speeds. 
     A still further advantage of the present invention is that the accessories will not need to change speed on wide open throttle shifts, which eliminates belt squeal problems associated with shifts from first to second gear under wide open throttle conditions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partial perspective view of the front of an engine with a pulley system in accordance with the present invention; 
     FIG. 2 is a side, sectional view of the speed limiting drive pulley assembly; 
     FIG. 3a is a sectional view taken along line 3a--3a in FIG. 2 and FIG. 3b is a sectional view taken along line 3b--3b in FIG. 2; 
     FIG. 4 is a partial front view of the pulley system with the front cover removed; 
     FIG. 5 is a view of the inside of the front cover of the pulley system; 
     FIG. 6 is a graph illustrating an example of the driven speed of an accessory versus the engine speed with and without the speed limiting drive pulley assembly; 
     FIG. 7 is a view similar to FIG. 2 illustrating a second embodiment of the present invention; 
     FIG. 8 is an exploded perspective view of the speed limiting drive pulley assembly of the alternate embodiment of FIG. 7; and 
     FIG. 9 is a view similar to FIG. 2 illustrating a third embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An engine 12 includes a crankshaft 14 having a speed limiting drive pulley assembly 16 attached to it, forming part of a pulley system 17. The drive pulley assembly 16 frictionally engages an accessory belt 18 mounted about it, which engages driven pulleys 20 on conventional engine accessories 22. 
     The speed limiting drive pulley assembly 16 includes a pulley member 24 which is affixed about a ring shaped pulley housing via a tolerance ring 27. The tolerance ring 27 serves to retain the pulley member 24 to the housing 26 and yet insulates the pulley member 24 from the temperature of the housing 26. 
     An outer ring portion of the housing 26 includes internal splines 28 splined to teeth 30 on a series of separator plates 32. This rotationally fixes the separator plates 32 relative to the housing 26. The separator plates 32 are interleaved with a series of friction discs 34 to form a wet clutch assembly 36. The friction discs 34 have friction paper on each side and radially spiraling grooves 35 on both of their paper faces. 
     The friction discs 34 also have teeth 37 which are engaged to splines 38 on a ring shaped damper mass (damper ring) 40. This rotationally fixes the friction discs 34 relative to the damper mass 40. The damper mass 40 is elastically attached to a hub 42 through damper spring packs 44, made up of steel leaf springs. Alternate spring packs 44 are pre-stressed in opposite directions (clockwise and counterclockwise) so that even at maximum damper excitation the springs never become completely unloaded, eliminating the possibility for unsprung free-play. A bushing 43 separates the damper springs 44 from the housing 26. A damper plate 46 is rigidly attached to the hub 42 and is located adjacent to the face of the damper mass 40 to provide viscous friction to the damper mass 40. 
     The hub 42 of the speed limiting drive pulley assembly 16 includes a bore 48 which is splined to the conventional engine crankshaft 14 and held in place with a screw and washer/puller 45. When removal of the hub 42 from the crankshaft 14 is desired, a retaining ring 41 serves to transfer the pulling force from the washer/puller 45 to the hub 42. A rear oil seal 49 seals between the housing 26 and the hub 42, while allowing the two to rotate relative to each other. 
     The clutch assembly 36 basically allows torque to be transmitted from the hub 42 to the pulley member 24. The torque transmitting capability of the clutch assembly 36 is proportional to the load it receives which presses its members together. 
     A front cover 50 includes a series of radial grooves 52 on its inside surface. It also includes radially extending fins 53 on the outside surface. The front cover 50 mounts to the housing 26, and a front oil seal 51 seals between the cover 50 and the hub 42 while allowing the two to rotate relative to each other. An O-ring 47 seals between the cover 50 and housing 26. 
     A piston 54 is located adjacent to the clutch assembly 36 in a mating bore in the housing 26, with a belleville spring (disc spring) 56 mounted between the piston 54 and the housing 26 to bias the piston 54 toward the clutch assembly 36. The belleville spring 56 provides a spring force that is relatively constant over the range of the piston travel, which reduces sensitivity to assembly tolerances and clutch wear. As an option, the spring force of the belleville spring 56 can be selected to produce an increased force when the clutch pack wears and a glazing of the friction discs 34 causes a reduction of the coefficient of friction. 
     During assembly, the piston 54 is initially installed into the mating bore in the housing 26 with the enclosed volume between the piston 54 and the bottom of the bore entirely filled with a volatile liquid such as a solution of water and ethylene glycol. A retaining ring 58 is installed to keep the piston 54 inside the bore and the piston 54 is forced to the bottom of the bore to force most of the liquid out through an evacuation passage 60 in the housing 26. The back face of the piston 54 is configured so that when the piston 54 is forced to the bottom of the bore and the belleville spring 56 is pressed flat, the enclosed volume is very small. The evacuatic passage 60 is then permanently sealed with a plug 62 to retain the small remaining amount of the liquid inside the bore and to prevent the entry of any air. When the piston 54 is released, the belleville spring 56 pushes the piston 54 outward against the retaining ring 58. The enclosed volume now contains only a small amount of the liquid and the vapor from the liquid. 
     The front face of the piston 54 has radially oriented fins 64 to give good heat transfer from oil, discussed below, in front of the piston 54 to the liquid behind the piston 54. The fins 64 also force the oil to rotate with the piston 54 so that the hydraulic force acting on the piston 54 is a function of the housing rotational speed, and not of the hub speed. The first separator plate in the series includes a member 66 that engages with the radial fins 64 of the piston 54 to prevent the piston 54 from rotating relative to the housing 26. 
     During assembly, the internal spaces of the pulley housing 26 are filled with oil and sealed therein when the front cover 50 is attached, so oil surrounds the entire clutch assembly 36. 
     During engine operation, the speed limiting drive pulley assembly 16 is basically direct drive at low engine speeds, i.e., the pulley member 24 rotates at crankshaft speed with no slip between the two. The belleville spring 56 biases the piston 54 against the wet clutch assembly 36 with enough force to allow torque to be transmitted through the pulley assembly 16 from the hub 42 to the pulley member 24 with no slip. 
     As the housing 26 is rotated faster by the crankshaft 14, the centrifugal force on the oil will cause it to push outward. As it pushes outward, it also presses against the piston 54 in the opposite direction of the belleville spring force. This reduces the amount of force with which the piston 54 pushes against the clutch assembly 36. Thus, as the housing 26 speeds up more, the force of the piston continues to be reduced, and the torque transmitting capability of the clutch assembly 36 lessons. The friction discs 34 can then begin to slip relative to the separator plates 32. 
     The torque transmitted during slipping operation conditions is through shearing of the fluid (oil) between the separator plates 32 and friction discs 34, which minimizes wear of the friction elements. By selecting the appropriate size and shape of piston 54 and spring force for the belleville spring 56, the oil pressure can completely cancel the force of the belleville spring 56 and allow the slipping at the desired rotational speed. 
     With increasing speed, the torque transmitting capability of the clutch assembly 36 diminishes such that with minimal accessory load, the pulley member speed is limited to a particular maximum speed while the hub 42 (and crankshaft 14) may rotate considerably faster. Line 68 in FIG. 6 illustrates the speed limit. 
     During clutch slipping, heat is produced in the fluid being sheared at the clutch faces. The difference in rotational speed between the grooves 35 in the friction discs 34, which rotate faster than the housing 26 and fling oil outward; and grooves 52 in the front cover 50, which rotates slower than the crankshaft 14 and allows the oil to travel radially inward, creates a pressure differential that pumps the fluid in a loop out from the clutch assembly 36, in past the front cover 50 and back to the clutch assembly 36. Further, the fins 53 on the outside of the front cover 50 reject heat from the oil in the grooves 52 to the air around the engine. 
     It is expected that in most applications and for most typical engine operations, the engine 12 will spend very little time at high enough speeds to cause slipping in the clutch assembly 36 and the small amount of slipping that occurs will not produce a large amount of heat. In some applications, however, the engine 12 may operate at high speeds for extended periods of time. The resulting prolonged slipping in the clutch assembly 36 will release a significant amount of heat, which will raise the temperature of the speed limiting drive pulley assembly 16. 
     There is a high temperature compensating feature to limit the maximum temperature at which the clutch operates. With increasing temperature of the oil, and hence the piston 54, the water vapor pressure behind piston 54 (in the housing bore with the belleville spring) acts on the piston 54 against the pushing action of the oil. This puts additional clamping load on the clutch assembly 36, increasing the housing speed at which the clutch will begin slipping. Since water vapor pressure versus temperature is highly non-linear, very little change in pressure occurs between cold and normal operating temperature and thus does not significantly affect the force applied to the clutch assembly 36 by piston 54, yet a large increase in force occurs between normal to hot temperatures, due to an increase of the pressure behind the piston 54 and adding to the clamping load on the clutch assembly 36. This serves to account for engine operating conditions where the clutch assembly 36 is heating up too much due to operation at high crankshaft speed for an extended period of time. While the accessories are now driven at a faster upper speed limit, possibly increasing noise and wear, the possible damage to the speed limiting drive assembly due to overheating is minimized. 
     The graph in FIG. 6 illustrates higher maximum speed cutoffs 70 above the base cut off speed 68 for an alternator based upon the effect of the heat compensating mechanism. 
     A separate function that the speed limiting drive pulley assembly 16 performs is as a crankshaft damper. The damper mass 40, being elastically connected to the crankshaft 14 through the damper springs 44 acts in a manner similar to a conventional rubber mass mounted to a crankshaft. However, the damper springs 44 are made of steel rather than rubber to resist the strain caused by the higher amplitude of motion of the smaller inertia mass that is required to allow it to be packaged inside of the housing 26. With this arrangement, both the speed limiting and damper functions are performed using less space than with two separate assemblies. 
     A second embodiment is disclosed in FIGS. 7 and 8 where the piston and belleville spring of the first embodiment are replace with a sealed bellows arrangement. For purposes of this description, elements in this embodiment that have counterpart elements in the first embodiment have been identified by similar reference numerals, although given 100 series numbers. 
     The hub 142 still drives the pulley member 124 through a clutch assembly 136. In this case, however, the axial force applied to the clutch assembly 136 is provided by a sealed bellows 176. The bellows 176 is located between the front cover 150 and clutch assembly 136. The free length of the bellows 176 is longer than its installed length, so that in assembly, the elastic compression of the bellows&#39; convolutions produces a spring force on the clutch assembly 136, enabling the clutch assembly 136 to transmit torque. Optionally, if needed, an additional spring, not shown, may augment the force. In this embodiment, damper plates 146 are located on both sides of the damper mass 140. 
     Oil in the housing 126 surrounds the bellows 176. Similar to the first embodiment, when the housing 126 rotates at high speed, centrifugal acceleration on the oil produces a pressure that acts to compress the bellows 176, reducing the clamping load on the clutch assembly 136 and allowing the clutch to slip. 
     Further, the interior of the bellows 176 is evacuated except for a small amount of fluid (such as water). This acts as a high temperature compensating feature as in the first embodiment. When the clutch 136 is slipping, heat is produced in the fluid being sheared at the clutch faces, as discussed above. As the oil passes from the clutch assembly 136 to the front cover 150 and back again, it flows around the bellows 176 and heats it to the oil temperature. If the temperature rises, the bellows 176, being evacuated of air, but containing the small amount of liquid, will try to expand due to an increase in vapor pressure. The bellows 176 will then press harder against the clutch assembly 136, reducing the slipping, and thus reducing the heat input to the system. At low and moderate temperatures, the torque transmitting capability is not significantly affected by changes of temperature, but as the temperature of the clutch 136 approaches damaging levels, the torque carrying capacity increases sufficiently to decrease the amount of slipping and resulting friction heat to minimize additional temperature increase. 
     FIG. 9 illustrates a third embodiment of the present invention, which is similar to the first embodiment except that the damping function of the damper mass and spring is not incorporated into the speed limiting assembly. For purposes of this description, elements in this embodiment that have counterpart elements in the first embodiment have been identified by similar reference numerals, although given 200 series numbers. 
     The damper mass and damper springs are replaced with a single ring member 240 that rotationally couples the hub 242 directly to the friction discs 234 without the elasticity of damper springs. In this case, there will be no torsional damping effect for the crankshaft, but the speed limiting effect of the clutch assembly 236 will still operate to protect the engine accessories and the high temperature compensating feature will limit the temperature in the housing. Although, the combination of the damper assembly with the overall speed limiting drive pulley eliminates the need for a separate conventional rubber crankshaft mounted damper, making the assembly that performs the two functions more compact. Further, the ring member 240 and the hub 242 can also be formed as one integral part, if so desired. 
     While certain embodiments of the present invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention as defined by the following claims.