Monthly Archives: March 2012

Effect of Varying Shaft Speeds

When the two yokes of the drive shaft are “in phase,” the speed of the transmission output shaft and the pinion shaft will be constant and the same if the line of drive of both shafts is uniform. The velocity of the drive shaft will not be constant, but this is unimportant as long as the velocity of the driving and driven shafts is uniform.

When the two yokes are “out of phase,” the rotational speed of the shafts will be uniform only if the shafts are operated in a straight line. When operating at an angle with the yokes “out of phase,” a conventional universal joint will cause the driven shaft to speed up and slow down each revolution. The number of turns per shaft will be the same, but the velocity of the driven shaft will fluctuate.

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Slip Joints

One method of lengthening or shortening the drive shaft is by means of a splined shaft coupling or “slip joint.” Fig.,46-11. This movement is less pronounced on a torque tube drive, but a slip joint is used at the one universal joint.

When the type of slip joint shown in Fig. 46-11 is used, it is possible to assemble it incorrectly. This results in annoying vibration. Such joints are usually marked for correct assembly. If they are not marked, make sure both yokes are in the same plane, or “in phase.”

wpg4 300x245 Slip Joints

 

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Drive Shaft Length

In addition to line of drive problems caused by angularity of the drive shaft, the distance between the transmission output shaft and the drive pinion shaft is subject to change.

This creates the need for some flexibility in the length of the drive shaft. In referring to the Hotchkiss drive in Fig.46-10, the front end of the drive shaft is attached to the transmission shaft at B. The rear end is attached to the transmission shaft at B. The rear end is attached to the drive pinion shaft at C. As the wheels move up and down, the drive shaft swings up and down in arc A-A around pivot B. At the same time, the pinion shaft C – being attached to the rear axle – swings in an arc D-D around the pivot where the spring is anchored to the frame E. Therefore, as the arcs A-A and D-D do not coincide, it will be necessary for the drive shaft to alternately lengthen and shorten as the wheels move up and down in relation to the frame.

 

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Control Arm Drive

In control arm drive, driving and braking forces are transferred to the front end of heavy-duty control arms, Fig.46-9. The torque transfer effect is similar to Hotchkiss drive, but coil springs are used at the rear rather than leaf type springs. Some cars use three control arms, most use four.

wpg3 300x189 Control Arm Drive

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Torque Tube Drive

In cars equipped with torque tube drive, the pushing action is at the front end of the torque tube, Fig.46-8. Only one universal joint is used in the drive line, at the front end of the drive shaft. In this case, the drive shaft is within a stout tube which is anchored to the axle housing. This torque tube does not permit the axle housing to twist when engine power is applied. The springs do not absorb any torque and are required only to cushion the ride.

With torque tube drive, the engine is usually mounted as low as possible in the frame at an angle with the rear end lower than the front. The object is to obtain a line of drive as straight as possible for power transmission, because power is always lost by any angularity in the drive line.

An unusual development in torque drive was engineered into early Pontiac Tempest cars. A comparatively slender drive shaft was used within a torque tube, fastened rigidly to the engine at one end and to a transaxle (combination transmission and axle) at the other end. The drive shaft bends of flexes as required to compensate for relative movement between engine and axle. Vibration and whipping is minimized by a rubber mounted ball bearing near the middle of the shaft.

wpg2 285x300 Torque Tube Drive

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Hotchkiss Drive

Two universal joints are used in the Hotchkiss type of drive to compensate for variations in road surfaces, load conditions and power application which cause changes in alignment between the transmission output shaft and drive pinion shaft. When the Hotchkiss type of drive is used, the vehicle is moved forward by the front end of the rear springs pushing against the frame, Fig. 46-7.

wpg1 300x137 Hotchkiss Drive

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Torque Transfer

This powerful torque force is transferred from the pinion shaft to the axle housing and from the axle housing to the springs, torque tube or control arms. In the Hotchkiss type of drive, it results in flexing and distortion of the spring. This distortion or “wind-up” of the springs will raise the pinion shaft at the front end, thereby decreasing the angle between the pinion shaft and the drive shaft.

 

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Torque Changes

However, when engine power is applied to the drive train, torque is developed in the driving wheels. This action creates further changes in the angularity of the drive line, Fig. 46-6.

wpg 300x292 Torque Changes

     When power is transmitted by the drive shaft, the pinion gear tries to turn the ring gear. Since the ring gear must turn the axle shafts (and the wheels, indirectly), it resists being moved. The pinion gear attempts to “roll around” the ring gear. Since it cannot, the pinion instead transfers the turning effort, or torque, to the axle housing. The magnitude of this torque is indicated by the tendency of the back end of the car to dip when power is suddenly applied to the driving wheels.

 

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