Page 262 - Failure Analysis Case Studies II

Failure Analysis Case Studies I1
D.R.H. Jones (Editor)
0 200 1 Elsevier Science Ltd. AI1 rights reserved 247

ANALYSIS OF A VEHICLE WHEEL SHAFT FAILURE

J. VOGWELL
Department of Mechanical Engineering, University of Bath, Bath BA2 7AY, U.K.
(Received 24 April 1998)

Abstract-This paper describes an investigation which was carried out on a failed wheel/drive shaft component
used on an unmanned, remotely operated vehicle for manoeuvring military targets. As many vehicles had been
manufactured and delivered to customers it was necessary to establish whether it was thought likely that more
failures might occur. A study of the broken shaft shows how vulnerable such a rotating component can be to
failure by fatigue, even when operating under steady conditions, if basic preventative design actions are not
taken. The analysis considers the effects of both transmission torque and weight (thus bending) upon stress
levels and assesses their individual affect on the breakage and upon any subsequent modifications needed to
improve the design. The drive shaft arrangement is compared with the feasible alternative of using a driven
wheel arrangement rotating on a stationary axle. Findings confirm the importance of recognizing in advance
the salient factors leading to fatigue and the necessity in paying adequate attention to detail during design and
manufacture if long service life is to be achieved. 0 1998 Elsevier Science Ltd. All rights reserved.
Keywords: Fatigue, fatigue design, machinery failures, vehicle failures.

1. INTRODUCTION

Having a wheel shaft which is directly driven from a power source (in this case, a battery operated
electric motor) using a timing belt transmission is a common means for driving small, relatively low
cost vehicles. The attraction of a rotating wheel shaft is that it can be made very simple because it
can accommodate the driven pulley, be used to mount the bearings and also attach a pair of wheels
all on a single component. Such a design has undoubted advantages compared to the alternative of
using a driven wheel hub arrangement located on an axle; as occurs, for example, with a chain
sprocket on a bicycle wheel. However, there is a fundamental difference in the two designs-with a
shaft, as it rotates, bending stresses alternate between tension and compression each revolution,
whereas with an axle, since it remains stationary, this is not the case. Even when travelling at
constant speed and carrying a steady load across level terrain, the alternating stress in a shaft can
lead to fatigue damage, especially when wheel diameters are small and consequently the number of
rotations becomes very significant. In the case of military target vehicles the problem is compounded
by the fact that they carry heavy batteries and must be protected by armour plating which adds
considerably to the weight carried and thus the potential for high bending moments along the shaft.
The design requirement is further complicated by additional factors, many of which are of a
variable nature. These include the effect of changing torque transmitted during acceleration of the
vehicle from start up to full speed and also abrupt breaking (these cffects will result in torsion and
thus changing shear stresses), travelling over uneven terrains without suspension will contribute
shock loading (further adding to fluctuating bending stresses). Consequently, the vulnerability to
fatigue damage is clearly very real and so it is essential to realise this and identify weak locations
and take preventative steps at the design and manufacturing stages.

2. THE WHEEL SHAFT DESIGN

The wheel shaft has been made from a stainless steel bar (grade 316) and has been turned down
to a central diameter of 20 mm with an overall length of 725 mm. A keyway slot is machined near
Reprinted from Engineering Failure Analysis 5 (4), 271-277 (1998)