Tensioning a drive belt

Abstract: A belt drive arrangement has a drive belt 14 which passes around and transfers rotation between a plurality of rotatable members (pulleys) 16, 26, 20, 22, 24. The drive belt path includes both a belt tensioner 18 and an idler pulley 22. The idler pulley has a rotation axis and is mounted to a fixed structure on a mounting axis 36 which is eccentric to the rotation axis. The idler pulley is provided with a lever 40 by which it can be rotated about the mounting axis to vary the position of the surface of the idler pulley which is in contact with the belt relative to the mounting axis, and thus to vary the tension in the belt.

Description: This invention relates to a method for setting the tension in a drive belt, and also to an arrangement for tensioning a drive belt. The invention is particularly (but not exclusively) applicable to drive belts in internal combustion engines, especially those used in motor vehicles. The term 'belt' when used in this specification includes all physical constructions for the belt which are used for the same function. Specifically the term 'belt' includes flexible rubber or rubber-like belts, whether flat, toothed or V-form, and chains with metal links. In a motor vehicle engine, a number of auxiliary devices are driven by the rotation of the engine crankshaft. Such auxiliary devices include the camshaft or camshafts, a water pump, an oil pump, an alternator, balance shafts and the like. The rotation of the crankshaft is generally transferred to these auxiliary devices by one or more belts which pass around a wheel on the crankshaft axis and then around a wheel on the drive shaft of the auxiliary device to be driven. In the case of the camshaft or camshafts, it is of very great importance that the camshaft rotation be synchronised with that of the crankshaft to achieve the correct engine timing. A toothed belt is normally used. It is necessary to maintain the correct tension in this belt, by which crankshaft rotation is transferred to the camshaft, both to ensure maintenance of the correct timing and to ensure a long life for the belt. Various arrangements are known for setting this tension, and it is conventional to include in the belt run an auto tensioner device which is spring loaded and which ensures that the belt is placed under a constant tension, irrespective of expansion and contraction of the belt itself and of the other engine components which can affect belt tension. When the belt is first installed, or when the belt is replaced in service, it is necessary to initially set the belt tension so that the auto tensioner is at the centre of its available travel so that the auto tensioner can thereafter make use of its full range of movement to maintain the correct belt tension. According to the present invention, there is provided a belt drive arrangement in which a drive belt passes around and transfers rotation between a plurality of rotatable members including a belt tensioner and an idler pulley, in which the idler pulley is mounted to a fixed structure on a mounting axis and has a hub with a sleeve mounted on the hub, the sleeve being freely rotatable about a rotation axis eccentric to the mounting axis and providing a surface with which the drive belt engages, the hub being provided with means by which it can be rotated about the mounting axis to vary the position of the surface of the idler pulley which is in contact with the drive belt and thus vary the tension in the belt, characterised in that the mounting axis is inside the perimeter of the hub and the means by which the hub may be rotated about the mounting axis is a lever permanently connected to the hub. By arranging an idler pulley in the belt path in this way, it is possible to rotate the pulley, on engine set up, to place the belt tensioner in the correct initial setting position so that the belt tensioner can thereafter react to factors tending to produce subsequent increases or decreases in the belt's tension. The hub is formed with a mounting axis which is eccentric to the rotation axis of the pulley. The hub also preferably carries means for rotating the hub. In a preferred form the hub has an eccentric aperture for a fastening bolt, the fastening bolt being intended to pass through the aperture and into a tapped hole in a fixed structure, for example an engine block or engine cylinder head. The hub also has a feature thereon which can be engaged by a tool to rotate the hub, before the fixing bolt is tightened to secure the hub in that position. The feature is a lever permanently connected to the hub, which may be engaged by a tool which allows the hub to be rotated under control. The invention also provides a method of setting the tension in a drive belt forming part of a belt drive arrangement, the drive belt arrangement being as claimed in any preceding claim, the method comprising the steps of: mounting the idler pulley to a fixed structure on a mounting axis which is eccentric to the rotation axis of the idler pulley, with the idler pulley lying in the belt path; releasing the mounting between the idler pulley and the fixed structure; rotating the idler pulley about the mounting axis until the tension in the belt tensioner is at a desired level; and securing the idler pulley in that position. The invention will now be further described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a schematic view of the end face of a motor vehicle engine which incorporates an arrangement in accordance with the invention; Figure 2 is a cross section through an idler pulley in accordance with the invention; and Figure 3 is a plan view of the idler of Figure 2. Figure 1 shows an end view of an internal combustion engine which has an engine block 2, a crankshaft 10, a camshaft 12 and a drive belt 14 connecting the shafts 10 and 12. The belt 14 passes around a first idler pulley 16, a belt auto tensioner 18, a toothed pulley 20 on the camshaft 12, an adjustable idler pulley 22 and a water pump pulley 24. Figure 1 also shows an auxiliary belt drive A extending around pulleys B and C, but this forms no part of the present invention. When the crankshaft 10 rotates (which it does when the engine is running) the belt 14 is driven to produce rotation of the camshaft pulley 20 and thus of the camshaft 12 together with rotation of the water pump pulley 24 which operates the water pump. As is conventional, the inside face of the belt 14 will be formed with teeth, whereas the outside run will be flat. The teeth are required to ensure that the rotation of the camshaft 12 is synchronised with that of the crankshaft 10, but no such synchronisation is required for the water pump 24, so this can be driven off the back flat side of the belt. It will be appreciated that any engine has to operate under a wide variety of different ambient conditions, and also that the engine includes a number of components of different materials having different rates of thermal expansion. It is important to ensure that the synchronisation (time) between the crankshaft 10 and the camshaft 12 is accurately maintained under all conditions, and this requires the belt tension to be kept constant. It is also necessary to keep the belt tension constant in order to ensure optimum belt service life. To keep the belt tension constant during engine operation, an auto tensioner unit 18 is provided. This unit, as is known, has an idler pulley 26 which runs against the flat back surface of the belt and is mounted on a swing arm (not visible in the drawing). The mounting of the pulley on the swing arm is such that the arm is spring biased in an anti-clockwise direction (referred to the position shown in Figure 1), and the spring biasing of the arm is selected so as to place a predetermined level of tension in the belt 14. The pulley 26 can swing over a range indicated at 28, with the pulley being shown in the middle of this range. When the engine is first built, it is important to set the belt so that the auto tensioner unit 18 is approximately in the middle of its possible range of movement. To do this an adjustable idler pulley 22 is provided. This idler pulley is shown in more detail in Figures 2 and 3 and comprises a pulley sleeve 30 mounted for free rotation on a hub 32. The sleeve 30 is toothed because, in the arrangement shown in the Figures, it engages with the toothed side of the belt 14. If the idler pulley was positioned in a different part of the belt run, the outer pulley sleeve 30 could be smooth. The sleeve 30 rotates relative to the hub 32 on bearings 34. As can be seen particularly in Figure 3, the hub has an eccentric mounting hole 36 and to mount the pulley, a bolt will be passed through the hole 36 and into a tapped hole on the engine block 2. When the bolt is tightened, the inner surface 38 of the hub 32 will come into frictional contact with the surface of the block 2 so that the hub will be locked in position. The hub 32 also has a rotation lever 40 which, as can be seen in Figure 2, is shrunk on to the outer periphery of the hub 32. The lever 40 has an actuation hole 42 into which an actuating tool can be placed. In use, when the engine is being assembled, the belt 14 will be placed around the appropriate pulleys. The auto tensioner unit 18 may be initially held out of the way so that it does not obstruct fitting of the belt. The pulley 22 will be secured by a bolt passed through the hole 36 but this bolt will not be tightened at this stage. The auto tensioner 18 will then be released, so that the pulley 26 moves under the influence of its spring loading against the belt 14 to put the belt into tension. It is then the assembly operator's job to set the auto tensioner so that it is initially in the middle of its travel, enabling it to compensate for both the expansion and contraction of the belt and other engine components. To do this the operator places a tool in the socket 42 and rotates the hub 32 to vary the distance between the centre of the bolt and thus the mounting axis of the hub 32 and the surface of the pulley sleeve 30 with which the belt 14 makes contact. Depending on the direction in which the lever 40 is turned, this will either increase or decrease the tension in the belt. When the auto tensioner idler pulley 26 is seen to be in its central position, then the bolt passing through the hole 36 is tightened to secure the axis of the pulley 22 permanently. The engine will then operate with the auto tensioner unit 18 maintaining the correct belt tension throughout the engine life. Throughout the autotensioner setting operation the camshaft pulley 20 is free to rotate on the camshaft 12 and the camshaft itself and the crankshaft 10 are held locked in their correct relative angular positions. Once the autotensioner has been set, the camshaft pulley is locked onto the camshaft 12 by tensioning a bolt 13 at the centre of the pulley. This bolt pulls a tapered bore on the pulley onto a matching machined surface on the camshaft itself to lock the components together. If the belt 14 has to be replaced at any stage, then the same procedure will be followed to set the position of the auto tensioner unit at the centre of its range when the new belt is fitted. Experience is shown that, surprisingly, very little effort is needed to rotate the lever 40, to put the necessary tension into the belt and to bring the auto tensioner into its middle position. A further feature of the invention is that the idler pulley 22 allows an engine built with slightly different components (for example a water pump with a different pulley) to still be part of the same belt drive arrangement because the idler pulley 22 can be used to compensate for the different dimensions arising in the belt path. Important features of the pulley construction are that whatever means are used for enabling the pulley to be rotated about the axis of the hole 36 should be low in height, and should allow good access to any tool designed to rotate the pulley about the eccentric axis.

Claims: A belt drive arrangement in which a drive belt (14) passes around and transfers rotation between a plurality of rotatable members (16,18,20,22,24) including a belt tensioner (18) and an idler pulley (22), in which the idler pulley (22) is mounted to a fixed structure (2) on a mounting axis and has a hub (32) with a sleeve (30) mounted on the hub, the sleeve (30) being freely rotatable about a rotation axis eccentric to the mounting axis and providing a surface with which the drive belt (14) engages, the hub (32) being provided with means by which it can be rotated about the mounting axis to vary the position of the surface of the idler pulley (22) which is in contact with the drive belt (14) and thus vary the tension in the belt (14), characterised in that the mounting axis is inside the perimeter of the hub (32) and the means by which the hub (32) may be rotated about the mounting axis is a lever (40) permanently connected to the hub (32). A belt drive arrangement as claimed in Claim 1, wherein the hub (32) is formed with the mounting axis. A belt drive arrangement as claimed in any one of Claims 1 or 2, wherein the hub (32) has an eccentric aperture (36) for a fastening bolt, the fastening bolt being intended to pass through the aperture (36) and into a tapped hole in a fixed structure (2). A belt drive arrangement as claimed in any preceding claim, wherein the lever (40) has a feature (42) thereon which can be engaged by a tool to rotate the hub (32), prior to securing the hub (32) in position. A method of setting the tension in a drive belt (14) forming part of a belt drive arrangement, the drive belt arrangement being as claimed in any preceding claim, the method comprising the steps of: i) mounting the idler pulley (22) to a fixed structure (2) on a mounting axis which is eccentric to the rotation axis of the idler pulley, with the idler pulley (22) lying in the belt path; ii) releasing the mounting between the idler pulley (22) and the fixed structure (2); iii) rotating the idler pulley (22) about the mounting axis until the tension in the belt tensioner (18) is at a desired level; and iv) securing the idler pulley (22) in that position.

Inventors: HAWES DAVID JAMES, HAWES, DAVID JAMES
Applicants: FORD GLOBAL TECH INC, FORD GLOBAL TECHNOLOGIES, INC.
Application year: 1997
Main classification: F16H7
IPC codes: F16H7