Mechanical Systems and Components Flashcards

A technician is servicing a piece of equipment, such as a walk-behind trowel, that requires a low output speed and high torque from the engine. Which of the following mechanical systems is used to achieve this?

Answer: A gear reduction system

A gear reduction system uses a set of gears to decrease the output speed from the engine while increasing the output torque. This is ideal for applications requiring high force at lower speeds.

What is the primary function of the lobes on a camshaft in a four-stroke engine?

Answer: To actuate the intake and exhaust valves at the correct time

The egg-shaped lobes on a camshaft are precisely designed to push against valve lifters or rocker arms, converting the camshaft's rotational motion into the linear motion needed to open and close the intake and exhaust valves. The shape and timing of the lobes dictate when the valves open, how long they stay open, and how far they open, which is critical for the engine's performance.

A customer brings in a single-cylinder engine that is vibrating excessively during operation. The technician has already confirmed the external mounting bolts are secure and the attached equipment (e.g., blade, pump) is balanced and undamaged. Which internal component is most likely responsible for minimizing this type of vibration?

Answer: The crankshaft counterweights

Crankshaft counterweights are designed to balance the rotational and reciprocating forces generated by the piston and connecting rod assembly. In a single-cylinder engine, these forces are not internally balanced by other pistons, making the counterweights critical for offsetting the inertial forces and reducing engine vibration.

Which type of bearing is best suited for applications with high radial loads in a compact space, such as in the connecting rod or as crankshaft main bearings in some small engines?

Answer: Needle roller bearings

Needle roller bearings use long, thin cylindrical rollers. This design distributes the load over a larger surface area, allowing them to handle high radial loads within a small, compact housing, making them ideal for applications like connecting rod ends and transmissions where space is limited.

A technician is having difficulty pulling the starter rope on a high-compression four-stroke engine. The resistance is extremely high. Which mechanism is designed to make starting easier by slightly opening a valve during the initial part of the compression stroke?

Answer: An automatic compression release (ACR)

An automatic compression release (ACR) system, typically mounted on the camshaft, uses a centrifugally activated mechanism. At low cranking speeds, it creates a small bump on the cam lobe that slightly opens the exhaust valve, bleeding off some compression and making the engine easier to turn over. Once the engine starts and RPMs increase, centrifugal force disengages the mechanism.

When diagnosing the cause of a bent crankshaft on a lawnmower, which of the following is the most common cause?

Answer: Impact with an immovable object like a rock or tree stump

A bent crankshaft is most often caused by a sudden, forceful stop of the rotating blade assembly. When the blade strikes an immovable object, the rotational inertia is transferred directly to the crankshaft, which can bend under the extreme force.