Form the design basis for screw joints

Abstract: This master thesis was performed at Scania CV AB in Södertälje at the engine
development department, NMKA and at Scania Latin America in Sao Paolo,
Brazil. Scania is a global company with operations in Europe, Latin America,
Asia, Africa and Australia. Scania is the world’s third largest manufacturer
for heavy trucks and heavy bus segment.

The constant development of new design solutions demands more stable
mounting in Scanias production. Assemblies with new screw joints with
different parameters affect the value of the clamping force for the joint
and consequently the final product. It is critical that the screw joint has
the right clamping force to assure that it fulfils its purpose. Large
deviations can result in total loss of clamping force and engine breakdown.
The purpose with this master thesis was to find the most important factors
that affect the clamping force and form the basis of how different screw
joints should be handled.

Knowledge about Scania screw joints were collected by working in the
production, observing drawings, a thorough literature study and by
interviewing representatives from different departments. The problems
regarding screw joints were found to be rather different at the departments
and the problems had been an issue for a longer period of time. The mapping
of screw joints resulted in a number of interesting screw joints with
different kinds of problems. A discussion about the interesting screw joints
was held in order to select joints to take a closer look at. By focusing on
area of interest, area of responsibility and resources, five screw joints
were selected, rocker arm shaft, main bearing cap, connecting rod and the
oil sump in both aluminum and steel. The chosen screw joints represented
specific types of screw joints on which the chosen parameters were tested.
The important parameters that were tested are driver speed, friction and
settlement.

The equipment that was used throughout the work was the Ultrasonic, which
measures the length of a screw with ultrasound. The authors feel that the
equipment has given reliable results and that it has been easy and
convenient to use during the tests since time consuming strain gages were
avoided.

The test results shows that the speed of the nut runner has a great
influence on the clamping force and torque. Depending on the nut runner and
which kind of screw joint that is examined the clamping force and torque
varies. The friction affects the result of the tightening substantially,
especially when tightening by torque. If torque and angle tightening is used
it is an advantage to keep the initial torque as low as possible but high
enough to keep the surfaces tight together before applying the angle. The
test results regarding settlement show that the settlement is extremely
dependent on the ingoing parts in the screw joint. Parameters such as screw
and clamped parts affect both the tightening procedure and degree of
settlement.

The authors recommendations are that a decision has to be made about what
information is important for each department. Furthermore, as the tightening
procedure is really important for the final clamping force it is advisable
to decide which department that is responsible of making the correct
tightening parameters of a specific screw joint in the production.

Hopefully this master thesis has woken an interest which in the future can
make a difference for an even more stable production and in the end an even
better quality at Scania CV AB.