1) What is the fundamental rule that is invariably applied to the design of a generic rotary power
transmission system and provide two reasons that justify its application.
2) Go on to briefly explain how this
rule finds use within the power-train of a front engined, rear wheel drive road vehicle. [6]
1 )Keep speed as high as possible for as long as possible over the length of the transmission
- Minimises size and weight of components
- Reduces cost
- Reduces the inertia within the drive train
2) In a conventional Power-train: - Engine drives power (high speed & low torque)
- Transmission (matches engine to road speed, two speed reduction, variable ratio)
- Final Drive (larger reduction, splits torque and changes direction in drive)
Name and highlight the key features of two mechanisms, found on a modern vehicle, that could employ epicyclic gear trains. [4]
1) Transmission Unit:
- numerous configurations available, employing several simple cyclic units
2) Mechanical Kinetic energy recovery system:
- Controls direction of power flow to and from the flywheel
What is the name often given to the transmission arrangement which two pairs of gears?
Why must the gear pairs posses equal numbers of teeth?
[4]
1) Co-axial
2) Must have gears of the same module
- Total number of teeth at each stage must be the same
Draw a spur gear tooth and label the 5 key features [5]
See notes
Why is it important to assess the magnitude of the first critical speed of a power transmission shaft using
two different approaches? [5]
- Rotating shaft becomes unstable at 1st critical speed
- Leads to a high amplitude of vibration
- Two methods of predicting speed, neither is exact:
- Rayleigh overestimates speed
- Dunkerly underestimates speed
1) State and briefly justify your choice, of the two areas on a spur gear tooth that could potentially
represent areas around which the tooth could fail when under load. [4]
2) Briefly outline the analytical approaches that permit assessment of the likelihood of failure for each case
identified. [4]
1) Contact stresses around the pitch point (normal load)
- based on Hertzian contact theory, stress levels are modified by correction factor
2) Cracking at root due to excessive bending stress (transmitted load)
- based on modified Lewis equation in which tooth represents simple cantilever subject to transmitted load at its tip
diagram needed
What are the main functions that are performed by a conventional clutch unit within an automotive
drive line? []
- Main function is to couple together two coaxial shafts within a transmission.
- Connect/disconnect two rotating shafts.
- Matching of shaft velocities should be progressive.
- When coupled the unit effectively behaves as a rigid coupling.
- Thereby permitting the transfer of power.
Explain why the thermal management of a clutch mechanism is as important as the torque capacity of
the unit. [4]
- Combination of sliding and normal load across interface develops heat within surface of rotor and ???
- If the interface becomes too hot, friction material can become glassy and its function can be compromised
Briefly explain why limiting the magnitude of the interface pressure is an important consideration to
take into account when designing a clutch. [4]
- Ability of friction material to generate/ carry normal load is friction material dependent so limits are imposed.
- Magnitude of pressure will also help limit or counter temperature rise
Briefly explain, using diagrams as appropriate, the mechanism that could lead to the propagation of a
crack at the root of a spur gear.
State the load responsible for this type of failure and indicate how you might choose to mitigate against such a failure.
[5]
Tooth behaves as a CANTILEVER BEAM under action of TRANSMITTED LOAD
MAXIMUM BENDING STRESS OCCURS AT ROOT - initiating a crack
A tensile side of loaded tooth
Can be prevented by increasing the tooth modulus
Describe briefly the problem of pitting, identify where you would typically find pitting and indicate the
load that is primarily responsible for its occurrence.
[5]
It occurs on the face of a tooth, involving material being removed from around the the pitch point
Driven by the NORMAL LOAD around he interface
Investigated through HERTZ constant analysis
Briefly outline the problem of shaft whirl and outline the procedure followed to estimate the first
critical speed of a power transmission shaft when treated as a single mass system.
[5]
- Power transmission shafts deflect when rotating
- At a specific speed the magnitude of this deflection becomes large, making the system unstable.
- This is the critical speed
- Single mass-spring system is the basis of this theory
State the reason that permits the methods of Rayleigh and Dunkerley provide differing
estimates of critical speed for multiple mass systems and indicate (along with your reason) which method
provides the upper limit.
[5]
- Rayleigh and Dunkerly methods allow multiple masses to be considered
- Each method predicts a different value
- Rayleigh overestimates hence is upper limit
- Dunkerly underestimates hence is lower limit
If shaft speed is greater than the 1st critical speed, how can the shaft be altered to address this?
- The shaft needs to be stiffened to increase the 1st critical speed.
- Increasing the shaft diameter, this also reduces the stress in the shaft.
- Decreasing the bearing centre distance
If a gear is to be keyed to a shaft, what condition must be met?
The minimum pitch diameter must be twice the shaft diameter
In order to mate, the pinion and gear must satisfy the following 3 conditions?
a) They must have identical module.
b) They must have identical pressure angle.
c) They must have identical addendum and dedendum
What is a compound gear train?
- At least one shaft carries two or more gears.
- The angular velocities of all gears are equal to that of the shaft on which they are mounted.
- Angular velocity of adjacent shafts fixed by the gear ratio.
What is the Addendum?
The radial distance from the pitch circle to the outside diameter
What is the Dedendum?
The radial distance from the pitch circle to the root circle
What is the Clearance?
The difference between the dedendum and the addendum.
Ensures that the tip of the pinion does not interfere with the root of the gear during mesh.
What is the pitch point and why is it significant?
Smooth relative motion is achieved by ensuring the common normal of the curves of the meshing teeth
pass through a common, stationary point.
This point is called the pitch point.
What is the significance of the pressure angle?
The angle influences the geometry of the involute curve that defines the tooth
profile.
What is the contact ratio?
The contact ratio, Cr, is the measure used to quantify the degree of overlap that exists within a given gear pair.
As one pair of teeth are leaving the mesh, then the following pair have made contact and have taken up their share of the load.
What are appropriate contact ratios?
Cr = 1.0 Only one pair of teeth in contact at any instant in time. Leads to poor all round performance and excessive tip loading of the teeth.
Cr = 1.2 Minimum target value of Cr, tooth load shared between 1 and 2 pairs of teeth.
Cr = 1.4 - 2 Majority of spur gears have a contact ratio in this range.
