CASE STUDY - TORQUE TIGHTENING

Presented below is a case history of a torque tightening problem experienced by a vehicle manufacturer.

The Problem

A manufacturer was experiencing problems with the fasteners securing a bracket supporting part of a rear suspension on a vehicle. There were two problems:

1. On some, but not all vehicles, the bracket was slipping resulting in fretting. The relative movement which was occurring between the bracket and the bolts was causing elongation of the bracket's holes and necking of the shanks of the bolts. This was happening even with the bolts being pre-applied with thread locking adhesive. Inspection of the bolts on failed units confirmed that the nuts were not rotating loose. The photo shows one of the necked bolts.

2. During assembly, on certain batches of bolts, a proportion were failing on initial torque-up. This was despite a torque wrench being used to ensure consistency of the torque value.

Background to the Problem
From test and analytical work completed during the initial design of the assembly, a clamp force from the fasteners of 105 kN was required to prevent slippage of the bracket from suspension induced loading. The bolts being used were four M12 strength grade 8.8 zinc plated fasteners. During the design stage it was realised that the effect of the thread adhesive was to increase the thread friction. Based upon a thread friction coefficient of 0.2 and an underhead friction coefficient of 0.14, a preload of 34 kN was calculated together with a tightening torque of 90 N-m. With four bolts being used, the clamp force of 136 kN was considered more than adequate for the application.

Due to the failures, the adequacy of the design of the assembly was re-assessed. Based upon this investigation it was revealed that:

1. The thread friction coefficient could vary between 0.14 and 0.25.

2. The underhead friction coefficient could vary between 0.10 and 0.18.

3. A prevailing torque of 7 N-m resulted from the frictional drag associated with the thread locking adhesive.

The Cause of the Problem
It was realised by the engineers that the problems they had been experiencing were as a result of frictional scatter not being accounted for at the design stage. Based on a torque wrench accuracy of 5%, 85.5 N-m would be the lowest value of torque applied to each of the bolts. Using this tightening torque with the highest values of friction and a prevailing torque of 7 N-m, they determined that the bolt's clamp force would only be 23.9 kN, under the worst case condition. This was significantly below that which the application required.

The Solution
To overcome the two problems the company's engineers re-assessed the basis on which they determined the bolt's tightening torque and the resulting clamp force. Briefly; they deduced that a higher strength grade of bolt was required. It was decided to use M12 strength grade 10.9 bolts, flange headed to provide resistance to vibration loosening and ease of re-assembly during maintenance work. To determine the tightening torque and resulting clamp force the engineers:

1. Determined the tightening torque using the lowest value of friction coefficients. A torque of 110.5 N-m was determined on this basis utilising 90% of the fastener strength due to the combined effects of tensile and torsional stresses.
2. This torque value was reduced by 5% (to 105 N-m) to allow for torque wrench inaccuracy. This torque value was to be specified so that even under adverse conditions, the bolts would not fail on initial torque-up.
3. Using the 105 N-m torque value and the highest anticipated friction coefficients, a clamp force of 30.4 kN was determined. This clamp force is the minimum value which could be anticipated based upon the worst case frictional conditions.

The piechart shows how the 105 N-m tightening torque is distributed within the fastener. (The chart is from the BOLTCALC program.)

Following a test programme, the revised fasteners and torque specification were introduced as a design change and as a service modification to vehicles in the field. No further problems were reported.