Study - Torque Tightening
Presented below is a case history of a torque tightening problem
experienced by a vehicle manufacturer.
Background to 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.
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
1. The thread friction coefficient could vary between 0.14
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.
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:
- 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.
- 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.
- 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.