The drive shaft is usually a hollow exposed shaft with a universal joint at each end. On trucks and also some large cars where the drive shaft would be too great in length to be made in one piece, a two-piece shaft is used. This type of shaft has a bearing known as a centre bearing attached to the chassis, which supports the rear end of the front shaft. Three universal joints are used. The joint behind the gearbox will correct any misalignment between the engine and the chassis, while the other two joints will correct changes in the angle of the rear shaft.
Cross Type Universal Joint
The purpose of a universal joint is to connect two shafts together while allowing the rotary motion and torque from one shaft to be transferred to the other shaft even though the two shafts may be at different angles to each other and in different planes. Most universal joints used on heavy vehicles are the Cardan, Hookes or cross type joint. A typical cross type universal joint consists of a cross or spider with four bearing journals on which are mounted four needle roller bearings. These bearings carry very high loads for their size while oscillating at very high speeds. The bearings are individually sealed to retain lubricant and to prevent contaminant entry.
The universal joint bearings are contained by forks of the drive shaft yoke and by the forks of the transmission or driving axle yoke. Companion flanges are often used instead of transmission and driving yokes. The cross type of universal joint has one major disadvantage. If the input shaft is revolving at a constant speed, the output shaft speed will not be constant. It will be faster at two points during each revolution, and slower at two other points during every revolution.
VARIATION IN ROTATIONAL SPEED CAUSED BY A CROSS TYPE UNIVERSAL JOINT
The amount of variation in speed depends on the angle between the two shafts and will be zero if the angle is zero. This variation is shown in the graph. The variation in rotational speed from a single cross type universal joint is cancelled out when a second cross type universal joint is used. This second universal joint is fitted to the other end of the drive shaft and the yokes of the two universal joints are In phase with each other (in line and parallel).
The purpose of the slip joint is to allow the drive shaft to vary in length while rotating and transmitting torque loads. The slip joint consists of a male splined tube shaft (stub) and a female splined yoke (sleeve). A threaded cap is used to retain the lubricant seal. As the sliding splines must both support the drive shafts and be capable of sliding under full torque loads, hardened and ground splines are used to provide adequate strength and wear resistance.
When a driveline has three or more universal joints, the driveline must be supported at one or more intermediate points by a shaft support (or centre) bearing. As a general rule, a heavy vehicle drive shaft should not exceed 1.75 m between universal joint centres. A centre bearing must be used if the drive shaft would exceed this length
PHASING AND ANGLES
To prevent noise when operating and to ensure a long service life for drive shaft components, correct drive shaft angles and correct phasing of the universal joints are very important.
When a drive shaft is in phase, the forks of the slip joint yoke and the forks of the fixed yoke of the tail shaft are in line. Normally there should be an alignment arrow stamped on the sleeve of the slip joint and on the tube to assure correct phasing. If these marks are not present they must be added before disassembly.
PARALLEL UNIVERSAL JOINT INSTALLATION
Drive Shaft Angles
The components of a driveline must be positioned so that the working angles of the universal joint between the transmission, driving axle and (if applicable) the interaxle are as equal as possible. The term “drive shaft angles” refers to the angle of each drive shaft and the operating angle of each universal joint in the driveline. It is important to remember that the variation in rotational speed produced by a single universal joint is cancelled out by using a second universal joint. This second universal joint is fitted to the other end of the drive shaft with the yokes of the two universal joints being parallel (in phase) with each other. However, this cancellation will only occur if the operation angles of the universal joints are equal. Unequal universal joint working angles will cause the driveline to operate with variations in rotational speed, which can result in noise, vibration and premature universal joint failure. Therefore it has been traditional to install the transmission and driving axle with the universal joints parallel with each other and, as shown in the diagram, this is known as a parallel installation.
NON-PARALLEL UNIVERSAL JOINT INSTALLATION
However, parallel installations have proved to be impractical in the case of some tandem-drive vehicles and some short wheelbase vehicles with a minimum, driveline length. In these instances, parallel installations would require the drive shaft to operate with very severe universal joint working angles.
To reduce the working angles; the drive axle is tilted upwards until the pinion centreline and transmission mainshaft centreline intersect midway between the universal joint centres. This is a non parallel (broken-back) installation as shown in the diagram.
DRIVE LINE ANGLES AND CALCULATIONS
Universal Joint Working Angle
To determine universal joint working angles, a spirit level protractor is required. When angles are read from the 0o mark, or horizontally as shown in the diagram, these are the readings, which must be recorded.
DRIVELINE ANGLE RECORDING CHART
- The universal joint working angles of a prime mover must be checked twice. Once with the semi-trailer fitted to the fifth wheel, and once without the semi-trailer.
- Inflate all tyres to the normal operating pressure.
- Park the vehicle in its normal attitude, or operating position, on a flat surface which is level in both lateral and longitudinal directions. This is not strictly necessary, but makes the calculations easier.
- Using a spirit level protractor, measure the angle between the face of the transmission output flange and the vertical. Record this angle, angle A, and whether it is “up” or “down”, on a diagram similar to the diagram below.
- Measure the angle of the forward drive shaft by placing the protractor on the tubing. Record this angle, angle B, and whether it is “up” or “down”.
- Measure the angle between the face of the forward axle input flange and the vertical. Record this angle, angle C, and whether it is “up” or “down”.
- Measure the angle between the face of the forward axle output flange and the vertical. Record this angle, angle D, and whether it is “up” or “down”.
- Measure the angle of the rear drive shaft by placing the protractor on the tubing. Record this angle, angle E, and whether it is “up” or “down”.
- Measure the angle between the face of the rear axle pinion flange and the vertical. Record this angle, angle F, and whether it is “up” or “down”.
JOINT WORKING ANGLE (JWA)
Using the recorded measurements, calculate the variation between the universal joint operating angles. This can be performed as shown in the following example.
On each drive shaft, the pair of universal joints should have working angles within 3o of each order.
Every universal joint has a designed maximum working angle, which must not be exceeded. In a parallel installation the universal joint working angles depend on speed as shown in Table 1.
Whenever the designed maximum working angle of a universal joint is exceeded, then the problem must be corrected.
DRIVELINE ADJUSTMENT AND RUN-OUT
Universal joint working angles may be adjusted by altering the pinion angle of the axle.
Depending on the type of axle and suspension used, this may be adjusted by:
- Changing the length of the torque arm.
- Shimming the torque arm brackets.
- Using wedge shims between spring seat and leaf spring.
After repairs to a damaged or worn drive shaft, it is essential that it be checked for run-out. This is done by mounting the drive shaft between lathe centres and taking dial indicator readings at the positions indicated in the diagram.
A maximum of 0.25 mm (0.010″) run-out is allowed in the centre of the tube or shaft; 0.115 (0.004″) mm at the neck of the slip joint stub shaft; and 0.125 (0.005″) mm run-out measured on the tube at a distance of 75 mm from the welds.
The critical speed of a drive shaft is the speed at which the rotational speed coincides with the transverse natural vibration frequency of the drive shaft. Running at critical speed will cause severe vibration, which is damaging to driveline components and connecting units.
A drive shaft running at its critical speed may “whip”. However, critical speed should not be a problem with drive shafts that are less than 1.7 m in length and which do not exceed 3000 rpm. Where driveshaft speeds greater that 3000 rpm are likely to be encountered such as I with transmissions having overdrive ratios, the drive shaft should not exceed 1 .25 m in length.