Brakes and Dynamometers Questions and Answers

Answer: B

The key's answer is 54.4 N.m.

The band wraps 180° (\(\pi\) rad) with \(\mu = 0.25\), so the tension ratio is:

\[ \frac{T_{1}}{T_{2}} = e^{\mu\theta} = e^{0.25\pi} = 2.193 \]

The key takes the 400 N pull as the tight-side tension T₁ and the anchored end as the slack side:

\[ \begin{aligned} T_{2} &= 400/2.193 = 182.4\ \text{N} \\ T &= (T_{1}-T_{2})r = (217.6)(0.25) \\ &= 54.4\ \text{N.m} \end{aligned} \]

Read strictly, the drum's rotation as drawn drags the band towards the anchored end, which would make that end the tight side: its tension would be 877 N and the torque 119.3 N.m. That value is not among the options; the key's 54.4 N.m is the torque with the 400 N side tight. 100.6, 22.1 and 15.7 N.m do not follow from either reading.

Answer: D

A dynamometer is used to measure power of an engine.

A dynamometer absorbs or transmits the engine's output while measuring the torque T at a known speed N, and the brake power follows as \(P = \frac{2\pi N T}{60}\). Rope brakes, Prony brakes, hydraulic and eddy current dynamometers all work on this principle.

Balancing to reduce vibration is done with balancing masses, fuel supply is regulated by the governor and the fuel system, and brakes in an automobile stop the vehicle rather than measuring anything.

Answer: A

The brake commonly used in motor cars is the internal expanding shoe brake.

In this drum brake, two curved shoes inside a drum attached to the wheel are pushed outwards against the drum's inner surface by a cam or hydraulic cylinder. Its compact, enclosed construction suits vehicle wheels, which is why it has long been the standard car brake, particularly on rear wheels.

Band brakes and band and block brakes are used on hoists, winches and cranes, and the external shoe (block) brake is typical of railway wagons and some machinery, not of cars.

Answer: B

The brake commonly used in trains is the shoe brake.

Railway vehicles use shoe (block) brakes: a cast iron or composite block is pressed against the tread of the wheel by air pressure through the brake rigging.

Band brakes and band and block brakes are used on hoists, winches and cranes, and internal expanding shoe brakes on road vehicles such as cars and motorcycles.