NCCER Certification Industrial Maintenance and Mechanical Trades Syllabus

Why must sheaves and sprockets be aligned before tensioning a belt or chain drive?

Misalignment causes uneven wear, excessive heat, noise, vibration, and premature failure of belts/chains and bearings. Both angular and parallel (offset) alignment must be correct.

What is 'chordal action' (chordal effect) in a chain drive?

The rise and fall of the chain as it engages sprocket teeth, causing speed variation and vibration. The effect decreases as the number of sprocket teeth increases.

What is the difference between a roller chain and a silent (inverted-tooth) chain?

A roller chain uses rollers that engage sprocket teeth, common for power transmission. A silent chain uses flat toothed links that mesh smoothly, running quieter at higher speeds.

What is the function of gears in a mechanical drive?

To transmit rotary motion and power between shafts while changing speed, torque, or direction of rotation through meshing teeth.

How does a gear speed reduction affect output speed and torque?

A speed reduction (driven gear larger than drive gear) decreases output speed and proportionally increases output torque. Speed and torque are inversely related at constant power.

What is the difference between spur gears and helical gears?

Spur gears have straight teeth parallel to the axis — simple but noisier. Helical gears have angled teeth that engage gradually, running smoother and carrying more load, but they produce axial thrust.

What is the purpose of a flexible coupling between two shafts?

To connect two shafts to transmit torque while accommodating small amounts of misalignment, end float, and shock/vibration that a rigid coupling cannot tolerate.

What is backlash in a gear set?

The clearance or 'play' between meshing gear teeth. Some backlash is needed for lubrication and thermal expansion, but excessive backlash causes noise and shock loading.

What are the two basic types of shaft misalignment?

Parallel (offset) misalignment, where shaft centerlines are parallel but offset; and angular misalignment, where the centerlines meet at an angle. Real misalignment is usually a combination of both.

What is 'soft foot' in shaft alignment, and why must it be corrected first?

Soft foot is a condition where a machine foot does not sit flat/evenly on its base (like a wobbly table leg). It must be fixed first because tightening the bolts distorts the frame and ruins the alignment.

What are the two common precision shaft alignment methods?

The dial indicator method (rim-and-face or reverse-dial) and the laser alignment method. Both are far more accurate than a straightedge/feeler-gauge rough alignment.

Why is thermal growth accounted for when aligning hot-running equipment?

Machines expand as they heat to operating temperature, changing shaft positions. Cold alignment is offset by calculated targets so the shafts become aligned once at operating temperature.

What is the difference between a centrifugal pump and a positive displacement pump?

A centrifugal pump uses an impeller to add velocity/kinetic energy to fluid (flow varies with pressure). A positive displacement pump moves a fixed volume per cycle (flow nearly constant regardless of pressure).

What is cavitation in a pump and what causes it?

The formation and violent collapse of vapor bubbles when suction pressure drops below the liquid's vapor pressure. It causes noise, vibration, and impeller erosion; insufficient NPSH (starved/restricted suction) causes it.

What does NPSH stand for and why does it matter?

Net Positive Suction Head — the suction-side pressure available above the liquid's vapor pressure. NPSH available must exceed NPSH required to prevent cavitation.

Why must a positive displacement pump never run against a closed discharge valve?

It keeps forcing a fixed volume of fluid, so pressure builds with nowhere to go, which can rupture piping or burst the casing. A relief valve is required on the discharge.

What is the difference between a reciprocating compressor and a rotary screw compressor?

A reciprocating compressor uses pistons in cylinders to compress air in pulses. A rotary screw compressor uses two meshing helical rotors to compress air continuously and smoothly.

State Pascal's Law as it applies to hydraulics.

Pressure applied to a confined fluid is transmitted equally and undiminished in all directions. This lets a small input force create a large output force via differing piston areas.

What is the formula relating force, pressure, and area in a hydraulic cylinder?

Force = Pressure x Area (F = P x A). For example, 1,000 psi acting on a 10 in² piston produces 10,000 lb of force.

What is the main difference between hydraulic and pneumatic systems regarding the working medium?

Hydraulics use incompressible liquid (oil) for high force and precise positioning. Pneumatics use compressible gas (air) for faster, cleaner, lower-force motion that cannot hold position precisely under load.

What is the function of an accumulator in a hydraulic system?

To store hydraulic energy under pressure (via gas or spring), providing supplemental flow, absorbing shock/pulsation, and maintaining pressure during peak demand or pump shutdown.

What is the difference between a gate valve and a globe valve?

A gate valve is for full on/off (isolation) service with low restriction when open; it is not for throttling. A globe valve is designed for throttling/flow regulation but has higher pressure drop.

What is the function of a check valve?

To allow flow in only one direction and automatically prevent reverse flow (backflow), protecting equipment such as pumps from backspin or backpressure.

What is the difference between a pneumatic and an electric valve actuator?

A pneumatic actuator uses compressed air (diaphragm or piston) and often has spring fail-safe. An electric actuator uses a motor and gears for precise positioning but responds slower and lacks inherent fail-safe.

What is the difference between a rising-stem and a non-rising-stem gate valve?

On a rising-stem valve the stem moves up as it opens, showing position and needing overhead clearance. On a non-rising-stem valve the stem only rotates and the gate travels on internal threads, so stem height stays constant.

What does the working load limit (WLL) of a sling mean?

The maximum load a sling or rigging component is rated to support under specified conditions in normal service. It must never be exceeded and includes a safety factor below breaking strength.

How does sling angle affect the tension in each leg of a multi-leg lift?

As the sling angle from horizontal decreases, the tension in each leg increases sharply. Low angles (e.g., below 30 degrees) create dangerously high leg loads, so larger angles are safer.

What are the crane hand signals for 'stop' versus 'emergency stop'?

STOP: one arm extended, palm down, swung back and forth horizontally. EMERGENCY STOP: both arms extended, palms down, swung back and forth. The operator must obey a stop signal from anyone.

What is the center of gravity of a load and why is it critical in rigging?

The point where a load's weight is balanced/concentrated. The hook must be positioned directly above the center of gravity so the load lifts level and does not tip or swing.

What is the difference between butt-weld, socket-weld, and threaded pipe joints?

Butt-weld joints are welded end-to-end for high-pressure/large pipe. Socket-weld joints insert pipe into a fitting then fillet-weld, for smaller high-pressure lines. Threaded joints screw together for low-pressure/small pipe and disassemble easily.

When bending tubing, what is the purpose of knowing the 'take-up' (gain/deduction)?

Take-up accounts for the material the bend consumes around the radius so layout marks land correctly, letting you place bends so finished leg lengths come out accurate.

Why is equipment leveling critical during machine installation?

A level, properly supported base ensures correct shaft alignment, even load distribution, and reduced vibration, and prevents frame distortion that leads to premature bearing, coupling, and seal failure.