Patent Application
20200132173
The invention relates to a tensioner, and more particularly, to a tensioner having a first swing arm and a second swing arm connected to a bracket, a damping strut having an asymmetric damping characteristic connected between the first swing arm and the second swing arm.
Most internal combustion engines comprise accessories such as power steering, an alternator and air conditioning to name a few. These accessories are typically driven by a belt. A tensioner is typically used to apply a preload to the belt in order to prevent slippage. The tensioner can be mounted to an engine mounting surface.
The engine may further comprise a start-stop system whereby the engine will shut down when the vehicle is not in motion, and when a driver command is received to proceed the engine will restart, usually by action of a motor-generator unit (MGU).
The start-stop function will tend to reverse loading on the belt. Hence, tensioners are available to accommodate belt load reversals. The tensioner may comprise one or more components which independently pivot in order to properly apply a required belt preload force in both belt drive directions. The tensioner may also be mounted directly to an accessory such as the MGU in order to save space in the engine bay.
Representative of the art is U.S. Pat. No. 9,795,293 which discloses a tensioner for tensioning a belt and includes first and second tensioner arms having first and second pulleys respectively. The first and second pulleys are configured for engagement with first and second belt spans, and are biased in first and second free arm directions respectively. A second tensioner arm stop is positioned to limit the movement of the second tensioner arm in a direction opposite the second free arm direction. The second tensioner arm stop is positioned such that, in use, the second pulley is engaged with the endless drive member while the second tensioner arm is engaged with the second tensioner arm stop throughout a first selected range of operating conditions.
What is needed is a tensioner having a first swing arm and a second swing arm connected to a bracket, a damping strut having an asymmetric damping characteristic connected between the first swing arm and the second swing arm.
The primary aspect of the invention is to provide a tensioner having a first swing arm and a second swing arm connected to a bracket, a damping strut having an asymmetric damping characteristic connected between the first swing arm and the second swing arm.
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 tensioner comprising a bracket, a first swing arm pivotally mounted to the bracket, a first pulley journalled to the first swing arm, a second swing arm pivotally mounted to the bracket, a second pulley journalled to the second swing arm, and a damping member connected between the first swing arm and the second swing arm, the damping member having a asymmetric damping characteristic.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention.
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.
Strut bushing 120 is press fit into an end of strut cylinder 110 so that flange 121 of bushing 120 engages inner diameter 111 of cylinder 110. The strut's inner components are assembled around rod 160. Spring supports 130 and spring 140 slide onto rod 160.
Bearings 241, 242 are pressed onto the hub of each pulley 211, 212 respectively, until each bearing bottoms out. Pulleys 211, 212 rotate with the inner raceway of the bearings. Dust caps 261, 262, are pressed onto the hub of each pulley 211, 212 respectively.
In normal mode crankshaft CRK is driving belt B. Belt B in turn drives the MGU pulley DP. In stop-start mode the MGU drives pulley DP which in turn drives belt B to drive crankshaft CRK, thereby starting the engine (not shown).
Bracket 290 has an arcuate form that encircles the driven pulley DP. Each tensioner sub-assembly 201, 202 is disposed opposite the other on bracket 290. Each sub-assembly pulley 211, 212 is coplanar with the other and with the driven pulley DP. Driven pulley DP projects within bracket 290. Bracket 290 is mountable to a driven device in surrounding relationship with a shaft of the driven device. Driven pulley DP is mounted to the shaft.
In order to increase fuel economy and efficiency, many automotive manufacturers are incorporating alternators with the capability to drive the accessory belt drive system (ABDS). Such alternators are commonly referred to motor generator units (MGU's) or belt starter generators (BSG's). These can be used to start the engine, charge the battery, and boost the vehicle.
During standard operation, the crank pulley drives the ABDS system. When this is the case, the tight side is the side of the belt that is entering the crank pulley, and the slack side is the side that is coming off of the crank pulley. However, when the MGU is used to drive the system (such as during starting), the tight side becomes the side of the belt entering the MGU, and the slack side is the side of the belt leaving the MGU. This is the opposite of the former situation when driven by the crank pulley.
The slack side of the belt is the side that requires a tensioner. Since the slack side of the belt changes during different modes of operation, a tensioner that can adapt to these changing conditions is needed in order to properly control belt tension.
The inventive tensioner controls belt tension on both sides in order to respond to the alternating belt slack side. It comprises two separate tensioners coupled by a mechanical damping strut.
As torque output by the driving pulley grows so does belt tension. The increase in belt tension tends to push the tensioner pulley on the belt tight side away from the belt path. Since the pulleys are linked by the strut, the tensioner pulley on the slack side is pulled into the belt path as the tight side pulley is pushed away. Since the strut can change length by elongating and contracting this motion does not occur in a 1:1 fashion between the two tensioners, that is, there is relative motion between the two tensioner pulleys 211, 212.
For example, if the tight side pulley moves 20°, the slack side might move 10°, while actual values may depend on drive geometry and other factors. Therefore, an increase in belt tension will tend to separate the tensioner pulleys relative to one another. As the pulleys move apart, the strut rod follows one pulley, and the spring and cylinder follow the other. This causes the spring to compress and the load in the spring to increase. The increasing spring load along with increasing rod/cylinder separation causes the damping wedges 150 to slide up the frustoconical portion 163 of the rod.
The frictional forces are denoted f1, f2, and f3. They are products of the two normal forces N1N2, and the spring force Fs respectively. Frictional force f2 is responsible for the majority of the damping, the other contributions are negligibly small. This is because the majority of the movement occurs between inner surface 114 and damping wedge 150, with the other two either not moving at all or barely moving. The movement causes energy to be dissipated as heat, thus damping the system. The magnitude of the spring force along with the angle of the frustoconical portion of the rod θ influences the magnitude of N1, which governs the magnitude of N2, which governs the magnitude of f2 according to the relationship f2=μ2N2 where μ2 is the coefficient of friction between the wedges and the cylinder bore. When moving in the loading direction, f2 is in the same direction as the spring force Fs and therefore is additive; that is, it works to increase tension in the damping strut beyond just the spring force.
On the other hand, as the torque output of the driving pulley decreases so does the belt tension. This causes the tensioner pulleys to move toward one another. The pulleys moving toward one another result in rod 160 plunging deeper into cylinder 110. This movement acts to decrease the load in spring 140.
When movement is in the unloading direction, the amount of wedging between wedge 150 and cylinder inner surface 114 reduces, and all of the frictional forces reverse direction as shown in
In an alternate embodiment the noted parameters of the device can be adjusted such that the frictional forces are substantially equivalent in both movement directions resulting in symmetric damping. Either asymmetric or symmetric damping can be used as required.
When the belt is being loaded by the driving pulley(s), belt tension is increased above a nominal level. This tends to reduce the probability of belt slip, dampens system vibrations, and reduces impulse magnitudes. Not only is this preferable for system performance, it is also advantageous to tensioner life—less violent movement equates to less wear.
When the driving pulleys in the system reduce torque and/or speed, the belt tension drops below the nominal value. During unloading, there is little to no probability of belt slip, so there is no reason to have the belt at the nominal tension. Allowing the belt to unload at lower than nominal tensions results in longer belt life than would occur without asymmetric damping.
While the damping in this system asymmetric, it is tunable as well. As noted, the magnitude of angle θ controls the magnitude of normal force N1 which in turn controls the magnitude of N2 and consequently f2, the primary damping force. Therefore, changing angle θ changes the amount of damping produced. Furthermore, alternate embodiments of the design can include multiple sets of wedges. In this way the amount of damping exhibited can be altered.
The mechanics behind multiple sets of wedges are similar to those of a single set system; however there are more steps involved. When multiple sets of wedges are introduced, both the number of frictional surfaces (and consequently frictional forces) increases, as well as the amount of frictional surface area. This increase in these two parameters leads to an increase in frictional damping. It is important to note that other alternate embodiments are not limited to a maximum of two sets of wedges as depicted in
In yet another embodiment, hydraulic or gas damping struts can be substituted for the wedge type struts described herein. Hydraulic damping struts and gas type damping struts are known in the damping arts.
Further, either symmetric or asymmetric damping can be applied to each of the struts in this embodiment. The configuration as to which dampers are symmetric or asymmetric can be varied to achieve a desired system response or characteristic.
A tensioner comprising a bracket mountable to a driven device in surrounding relationship with a shaft of the driven device, a first swing arm pivotally mounted to the bracket, a first pulley journalled to the first swing arm, a second swing arm pivotally mounted to the bracket, a second pulley journalled to the second swing arm, a damping strut member connected between the first swing arm and the second swing arm, the damping member having an asymmetric damping characteristic, and the damping strut member comprises a body and a cooperating rod, a first wedge member fictionally disposed between a frustoconical portion of the rod and a body inner surface, and a spring urging the first wedge member into frictional engagement with the frustoconical portion and the body inner surface.
Although forms of the invention have 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. Unless otherwise specifically noted, components depicted in the drawings are not drawn to scale. Further, it is not intended that any of the appended claims or claim elements invoke 35 U.S.C. 112(f) unless the words “means for” or “step for” are explicitly used in the particular claim. The present disclosure should in no way be limited to the exemplary embodiments or numerical dimensions illustrated in the drawings and described herein.
