This assignment relates to the analysis of power transmission and vibration. There
are four questions. Answer all questions.
The assignment forms Element 2 of the module’s assessment criteria that covers
Learning Outcomes 1, 4, 5 and 6 as indicated below.
Knowledge and Understanding
1 Demonstrate a detailed understanding of mechanical engineering principles.
Cognitive and Intellectual Skills
2 Analyse and apply solutions to a variety of loading systems and the behaviour
of loaded beams and cylinders.
3 Understand the effects that constraints have on the performance of a dynamic
mechanical system through the applications of force and energy transfer.
4 Analyse and apply the dynamics of geared systems and power transmission
system elements for a variety of engineering problems.
5 Analyse natural and damped vibrations within translational and rotational mass-
spring systems.
Key Transferable Skills
6 Act with increased autonomy, with reduced need for supervision to assess and
suggest limitations to a range of mechanical systems.
A gear train is to be designed to transmit power from shaft 1 to shaft 2 with
the following conditions:
(i) Shaft 1 (driven by an electric motor) rotates at 100 revs min–1
.
(ii) Shaft 2 is to rotate at 400 revs min–1
in the opposite direction to shaft 1
against a load of 200 Nm.
(iii) The centre of shaft 1 is 300 mm from shaft 2.
(iv) The minimum number of teeth on any gear is 15. All gears must have a
multiple of 5 teeth and have a module of 2 mm.
(v) The maximum number of gears permissible is 4 gears and the
diameter of the largest gear must be minimized.
(vi) The centres of the gears should lie on a line which is as close to
straight as possible.
(vii) All gear shafts have a frictional resistance of 5 Nm.
Carry out the following:
(a) Design a gear train which satisfies the above criteria. Use a sketch to
illustrate your design, label the gears A, B, C, etc. from the driver to the
driven gear and state the number of teeth on each gear.
(b) Determine the input power required at shaft 1.
(c) Specify the efficiency of the gear train as a percentage.
(d) Determine an equation for the efficiency of the gear train in terms of the
load (torque) on shaft 2 with all other factors remaining constant.
There are two jack screw assemblies both with an ISO Metric Trapezoidal ‘Tr
16 x 3’ screw, one single-thread and the other double-thread. Each assembly
raises and then lowers a load of 450 kg.
A ball-thrust bearing forms part of each assembly and therefore collar friction
is negligible.
The coefficient of sliding friction for both screws is 0.08.
(a) For each assembly, determine:
(i) the pitch, pitch diameter, lead and lead angle;
(ii) the running torque required to lower the load;
(iii) whether they are back-driveable or self-locking?
(b) Which assembly is the most efficient in lifting?
(c) One way of ensuring that a lifting device can be made to self-lock is
to use a thrust collar rather than a ball-thrust bearing. For any of the
assemblies found to be back-driveable in (a)(iii), what is the
minimum collar diameter required to ensure it/they would self-lock?
