Question 1 [9 marks]

a) Briefly describe with the aid of simple sketches in your answer booklet, what you think are

the most likely types of gearing inside each of the gearboxes shown below (i) to (vi). Only

those shafts that emerge from the gearbox have been shown. In some cases other shafts may

exist inside the gearbox.

(i) (ii)

(iv) (iii)

(v) (vi)

ωi =

1000

RPM

ωo = 340

RPM

ωo = 340 RPM

ωi =

1000

RPM

ωi = 1000 RPM

ωo = 1000

RPM

ωi = 1000 RPM

ωo = 500 RPM

ωi = 1500 RPM ωo = 20 RPM

ωo = 100

RPM

ωi = 1000

RPM

b) (i) The Lewis equation and modern refinements of it are based on the determination of a

particular stress in gear teeth. Explain with a sketch the location and nature of this stress.

(ii) What damaging stress condition is not covered by the Lewis equation and those

equations derived from it?

c) (i) In your answer booklet complete the following statement of the kinematic law of

gearing, which is missing approximately 12 words:

"If the line drawn normal to the contacting surfaces of two rotating bodies

…………………………………………………………………………………………………

…………………..., the angular velocity ratio of the two bodies will remain constant."

(ii) Name two possible shapes of gear teeth that satisfy the law of gearing.

d) What are two important operational advantages of helical gearing compared to spur

gearing?

e) What are herringbone gears, and what special attribute do they offer the designer? Use a

sketch to explain.

Question 2 [ 5 marks ]

a) Make an isometric (3D) sketch of a Hooke's joint. Label the principal elements.

b) What problems occur if a single Hooke's joint is used but which can be eliminated if a pair

of them is used appropriately?

c) For the shaft system shown schematically below, which runs between two parallel shafts

with a given offset, sketch in your answer booklet an appropriate arrangement incorporating

two Hooke's joints and an intermediate shaft.

X

___________Y

d) Explain in what circumstance it would be necessary to fit a sliding spline joint in the

situation described in part c).

Question 3 [5 marks ] The horizontal shaft of a winch drum, which has a brake drum attached to it, is supported by two

spherical roller bearings mounted in plummer blocks (also known as “pillow” blocks). These

plummer blocks are bolted to a welded steel frame. The left hand bearing is fully located; the other

is axially floating on its outer race.

(a) Sketch a cross-section of one of the bearings and its plummer block. Label the:

• inner race

• outer race

• spherical rollers

• plummer block housing

(b) Sketch a simplified schematic of the mounting system showing both bearings, and with

heavy dots indicating the restraints, for the locations where they are used, for each of the inner

and outer races.

(d) Give two reasons why this mounting system is appropriate for this application.

Question 4 [ 8 marks ] The short shoe brake shown in the figure below is applied by the spring and released by a hydraulic

cylinder (not shown).

(a) Determine, with the aid of appropriate free body diagrams, the spring force required to

produce a braking torque of 175 Nm given that the co-efficient of friction is 0.35.

(b) A shoe brake may be “self actuating to some extent”. Is this the case for either of the caliper

arms shown? Explain your answer by reference to your free body diagrams.

(c) For the most heavily loaded shoe, determine the minimum possible width of shoe

(i.e. measured normal to the page) if the average pressure on the projected area of the shoe is not to

exceed 500 kPa.

Question 5 [8 marks] DO THIS QUESTION OR QUESTION 6 (a) The figure below shows the hub of an idler gear into which plain bearing bushes have been

pressed. When the hub is assembled and mounted on the shaft the permitted minimum and

maximum values of axial clearance “C” are 0.15 and 1.05mm respectively. Determine the

upper and lower limits of size the hub length “H” may have given that the distance between

shoulders “S” is 150.00 ± 0.15mm, and bush face-thickness “B” is 9.00 ± 0.08mm.

(b) In the operation of oil lubricated plain bearings we distinguish between two main regimes of

lubrication. State what they are and describe the function of the lubricant in each. Use

sketches to facilitate your explanation.

(c) Two important properties of the materials used in plain bearings are “compressive strength”

and “embeddability”. Discuss what is meant by these terms; why they are important; and

what conflict, if any, exists between them.

(d) On earth scooping machinery the pin joints on the linkage working the scoop obviously

need bearings of some sort. Given that shock loadings occur would you use plain bearings

or deep groove ball bearings in this application? Explain why.

Question 6 [8 marks]

DO THIS QUESTION OR QUESTION 5

(a) Referring to the figure below, the load is applied to the flat bracket via a pin in the hole.

Discuss with the aid of sketches the various modes of failure that a designer should consider

when determining the necessary proportions of the pin and the plate near the hole.

(b) The flat bracket is to be attached to a vertical surface using a pattern weld run as

shown in Figure 3. The weld metal will be laid as an equal legged fillet weld.

(i) Determine the location and magnitude of the maximum load on the weld (in N/mm

of weld run).

(ii) Suggest a size of the weld leg based on your findings assuming that the allowable

“resultant” stress on the throat of the weld is 0.33 x the ultimate strength of the weld

material, and that the weld material has an ultimate strength of 375 MPa.

91,000 N

320 mm

160mm 300 mm

The following formulas are provided, as are geometry factors for weld analysis on page 5:

Loading type Force per mm run of weld

Direct Shear w

A

ForceShear

L

V =

Normal Force w

A

ForceNormal

L

V =

Torque w

J

rT

L

V . =

Bending Moment w

Z

M

L

V =

Weld geometry factors. Source: Robert L Mott “Machine Elements in Mechanical Design”

Weld dimensions Bending Torsion