Intrinsic Design Faults in Truck Propeller Shafts

Intrinsic Design Faults in Truck Propeller Shafts
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Avoidable Design Faults in Truck Propeller Shafts

The fitment of needle rollers in the bearing cup into the yoke is not a calibrated press fit process. It is manual fitment. A tight fit causes high friction and fast wear on the race. Mating surfaces of the shaft splines are machined. Sliding friction will impede free to and fro movements. The propeller shaft assembly wobbles and causes heavy vibrations, and the entire drive system becomes damaged beyond repair.

Overloading a Major Problem

Background: In 1960, TELCO [now known as Tata Motors] finalized an agreement with Daimler Benz AG of Germany for manufacturing trucks in India. Telco had a seven-year contract with Daimler Benz. Those models were developed in World War II, and these trucks were robust. The real reason for bad service records was excessive overloading of the trucks. Our main roads and cross country roads were in a very bad state. Telco started manufacturing indigenous models of trucks in 1980. (Please see photo A below.)

We have good roads now, but the practice of overloading continues. The fuel cost in India is nearly $1 per liter. The road taxes are also high. This is the reason why truck drivers overload their vehicles- to earn more money to meet their ends.

Design and Failure Mode of Propeller Shafts

The propeller shaft transmits drive power from the engine via a gear box to the differential crown wheel pinion. It propels the vehicle at variable torque and variable rotational speeds in a floating state in relation to its rapid positional changes whilst adjusting spontaneously to changes in its length and its 3D planar motion. The three-directional motion of the propeller shaft is seriously restricted when the vehicle runs on the torture track or on irregular cross country roads. The universal joints have U-fork couplings with universal spider arms on needle roller bearings.

Imagine what happens to human shoulder ball joints if there is inertia in its free motion due to some deformation or localized physical constraint. We feel immense pain and consequent swelling in the muscles.

The propeller shaft is not able to express itself for any possible intrinsic inertia in its universal joints system. The propeller shaft wobbles, and the entire floating drive system eventually fails. These incidents are reflected in cumulative failures in subassembly components of the gear box, needle roller bearings of the prop shaft, wobbling of the prop shaft, and excessive vibrations.

These intense vibrations result in bearing noise in the gear box and pinion taper roller bearings which eventually damages the oil seals at the differential end, resulting in breakage of gear teeth or excessive wear. Not only is the deficiency obvious in the prop shaft and its final assembly, but its online testing is significantly irrelevant at predicting the failure. If one feels the free motion of the universal joints for the lateral movement of splined shaft of the tested and accepted prop shaft assembly, its free movements are seldom noticed.

Manufacturing Improvements for Propeller Shafts

A propeller shaft assembly is centrifugally balanced on a Schenk balancing machine. The electronic proximity sensors indicate eccentricity of the shaft. Steel strips are spot welded to the shaft at an appropriate location to avoid eccentricity. The propeller shaft has machined grooves of square profile. The mating surfaces are rough. It causes inertia in free axial movement of the shaft. The mating surfaces of the grooves are rough.

If the prop shaft assembly has genuinely free joints, it becomes difficult to reckon the offset in the balancing operations. The bearing cups with its needle rollers are press fitted into the spider arms to a blind location against a snap ring registered in its groove. The needle rollers get partially jammed into its position. Any excessive force applied on the bearing cup may cause the needle roller ends to contact the bearing cup inner end surface, and the surface contact restricts free rotational swivel motion of the spider arm. This is blind fitment.

Solution: The Volvo design of the prop shaft reduces friction and inertia. It solves many problems.

The Volvo-designed propeller shaft has semicircular grooves. The grooves are induction hardened and ground finished with a smooth surface. The grooves mating surfaces have steel balls for free to and fro motion. This is a proven design with a long service life. Volvo has excellent transportation services and has profound reputation for safety and environmental care.

Photos presentation : A- Telco indigenous 1980 models , B - Typical service failure