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newSchoolWorks.com › VoTechWorks › Manufacturing & Marine › Industrial Maintenance › Pumps: Selection, Installation, Operation, and Maintenance › Lesson 9
MFG-INDU16-04 · Lesson 9 · 13 clock hours · demo

Positive-Displacement Pumps: Gear, Vane, and Diaphragm

Objective. Students will demonstrate understanding of positive-displacement pump operation by disassembling a gear pump and a diaphragm pump, measuring internal clearances, identifying wear, and describing how a relief valve protects a PD pump circuit.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed engineer or certified journey-level tradesperson or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

VoTechWorks program approval and accreditation are still in progress. Credentials are issued only by the certifying bodies named in each program, after their own exams.

What Makes a Pump "Positive-Displacement"?

A positive-displacement (PD) pump moves a fixed volume of fluid with every rotation or stroke, regardless of the pressure on the outlet side. Unlike centrifugal pumps, which can "slip" and deliver less flow as backpressure rises, a PD pump will continue pushing fluid until something gives way — a pipe, a seal, or the pump itself. That mechanical fact is why a relief valve is not optional on a PD pump circuit; it is a required safety device.

Gear Pumps

An external gear pump uses two meshing spur gears rotating inside a close-tolerance housing. Fluid is trapped in the spaces between gear teeth and the housing wall, carried around the outside of each gear from inlet to outlet, and then squeezed out as the teeth mesh back together. The pump delivers smooth, continuous flow proportional to shaft speed.

Internal clearances are everything in a gear pump. Two clearances matter most:

Measure both clearances with a feeler gauge: insert blades of increasing thickness until the gauge just drags. Record the measurement, then compare it to the manufacturer's specification table. If clearance exceeds the wear limit, the pump needs new gears, a new housing, or both.

Safety during disassembly: Follow lockout/tagout (LOTO) procedures before opening any pump. Wear safety glasses and nitrile gloves. Fluid under residual pressure can spray when fittings are loosened. Crack the fitting slowly, and have rags ready.

Vane Pumps

A vane pump uses a rotor with radial slots. Spring-loaded or pressure-loaded vanes slide in and out of the slots as the rotor turns inside an eccentric cam ring. The vanes seal against the cam ring, forming expanding chambers on the inlet side and shrinking chambers on the outlet side. Vane pumps are sensitive to contamination — a single piece of hard debris can score the cam ring or break a vane. Filter condition is therefore a first-line maintenance check.

Diaphragm Pumps

A double-diaphragm pump uses two flexible diaphragms connected by a common shaft (or driven by air pressure alternating between chambers). As one diaphragm pulls back, it creates suction and draws fluid through an inlet ball check valve. As it pushes forward, it forces fluid out through an outlet ball check valve. The second diaphragm works the opposite stroke simultaneously, giving near-continuous flow.

Diaphragm inspection criteria:

Any of these conditions means the diaphragm must be replaced. When installing new diaphragms, consult the manufacturer manual for torque sequence; uneven bolt torque causes the diaphragm to seat off-center and fail early.

Ball valve inspection: The check valves in a diaphragm pump are simple ball-and-seat assemblies. Look for flat spots, pitting, or debris embedded in the seat. A leaking check valve reduces flow and can cause backflow.

The Relief Valve: Circuit Protection

Because a PD pump generates pressure proportional to the resistance it sees, a closed or blocked outlet will cause pressure to rise until a mechanical failure occurs. The relief valve is installed on the pressure (outlet) side of the pump, before any isolation valve, and is set to open at a pressure below the rating of the weakest component in the circuit. When pressure reaches the set point, the valve opens and routes fluid back to the reservoir (tank), limiting system pressure.

Relief valve placement rules:

1. The relief valve must be between the pump outlet and the first isolation valve — never downstream of an isolation valve that could be closed to trap pressure.

2. The valve return line must go back to the tank, not to the pump inlet.

3. The relief valve must be sized to pass the full pump flow without excessive pressure rise.

Why bypassing or blocking the relief valve is a serious hazard: Without pressure relief, a blocked line can pressurize to the point of rupturing hoses, blowing seals, or bursting the pump casing. Fluid injection injuries — where hydraulic fluid penetrates skin — are a recognized industrial injury requiring immediate medical attention. Never defeat, cap, or adjust a relief valve out of its rated range.

Circuit Schematic — Required Elements

A basic PD pump hydraulic circuit schematic must show, in order:

1. Reservoir (tank)

2. Suction filter or strainer

3. PD pump

4. Relief valve (teed off the pressure line, return to tank)

5. Pressure filter

6. Isolation valve (if used)

7. Actuator (cylinder or motor)

8. Return line to tank

Drawing and labeling this schematic is a core skill because troubleshooting always starts with understanding the circuit.

Lab Safety Summary

Key terms9

Positive-displacement pumpA pump that delivers a fixed volume of fluid per revolution or stroke regardless of outlet pressure, meaning outlet pressure rises until the system resistance is overcome or a relief valve opens.
Radial (gear-to-housing) clearanceThe gap between the tip of a gear tooth and the pump body wall; excessive clearance allows fluid to slip from outlet back to inlet, reducing volumetric efficiency.
Axial (end-plate) clearanceThe gap between the face of the gear and the pump end plate or cover; wear here allows fluid to bypass internally between pressure and suction sides.
Feeler gaugeA set of thin, precision-ground metal blades used to measure small gaps or clearances by inserting blades until a defined drag is felt.
Check valve (ball type)A one-way valve using a ball pressed against a seat by spring force or pressure differential; allows flow in one direction only and prevents backflow.
Diaphragm delaminationSeparation of the internal fabric reinforcement layers within a flexible pump diaphragm, causing loss of mechanical strength and imminent failure.
Relief valveA normally closed pressure-limiting valve installed on the outlet side of a PD pump that opens and routes flow back to the reservoir when system pressure reaches the valve's set point, protecting the circuit from overpressure.
Lockout/Tagout (LOTO)An OSHA-required procedure (29 CFR 1910.147) in which energy sources are isolated and locked in the de-energized state before maintenance work begins, preventing unexpected startup or release of stored energy.
Volumetric efficiencyThe ratio of actual pump output flow to the theoretical flow calculated from pump displacement; reduced by internal leakage caused by worn clearances or damaged check valves.

Worked examples4

1. A gear pump manufacturer specifies a maximum allowable gear-to-housing clearance of 0.004 in. You insert a 0.003 in feeler gauge blade and it passes with a light drag; a 0.004 in blade will not enter. Is the clearance within specification?
  1. Step 1 — Identify the measured clearance. The largest blade that passes with a light drag is 0.003 in, so the clearance is approximately 0.003 in.
  2. Step 2 — Compare to the specification limit: 0.004 in maximum.
  3. Step 3 — Decision: 0.003 in < 0.004 in, so the clearance is within the manufacturer's wear limit.
Answer: Yes. The measured clearance of approximately 0.003 in is less than the 0.004 in maximum; the pump is within specification.
2. The same pump's end-plate clearance spec allows a maximum of 0.003 in. Your feeler gauge shows the 0.003 in blade passes freely and the 0.004 in blade also passes with only light drag. What is your finding and what action is required?
  1. Step 1 — The 0.004 in blade passes with light drag, meaning clearance is approximately 0.004 in.
  2. Step 2 — Compare to the 0.003 in maximum specification: 0.004 in > 0.003 in.
  3. Step 3 — The clearance exceeds the wear limit. Document the out-of-spec measurement on the inspection form.
  4. Step 4 — Recommend corrective action: replace the end plate, gears, or both per the manufacturer service manual before returning the pump to service.
Answer: Out of specification. The end-plate clearance of approximately 0.004 in exceeds the 0.003 in limit. The pump must not be returned to service until worn components are replaced.
3. A double-diaphragm pump is rated for a maximum of 100 psi. The system relief valve is currently set at 120 psi. By how many psi is the relief valve set above the pump rating, and what is the corrective action?
  1. Step 1 — Calculate the overpressure amount: 120 psi (relief setting) − 100 psi (pump rating) = 20 psi above the pump rating.
  2. Step 2 — The relief valve should be set at or below the lowest-rated component in the circuit. A 20 psi excess means the relief valve will not protect the pump from overpressure damage.
  3. Step 3 — Corrective action: Following LOTO, adjust the relief valve set point to at or below 100 psi (or the next lowest-rated component if less than 100 psi) per the manufacturer's adjustment procedure.
Answer: The relief valve is set 20 psi above the pump rating. It must be adjusted downward to at or below 100 psi to provide proper overpressure protection.
4. During feeler gauge measurement of a gear pump, a technician records gear-to-housing clearance as 0.0025 in and end-plate clearance as 0.0035 in. Specifications are: gear-to-housing max 0.004 in, end-plate max 0.003 in. Which clearance(s) are within spec and which require action?
  1. Step 1 — Gear-to-housing: 0.0025 in measured vs. 0.004 in maximum. 0.0025 < 0.004 → within specification.
  2. Step 2 — End-plate: 0.0035 in measured vs. 0.003 in maximum. 0.0035 > 0.003 → exceeds specification.
  3. Step 3 — Record findings on the inspection form. The gear-to-housing clearance is acceptable; the end-plate clearance is not.
Answer: Gear-to-housing clearance (0.0025 in) is within the 0.004 in limit — no action required. End-plate clearance (0.0035 in) exceeds the 0.003 in limit — worn end plate or gears must be replaced before the pump returns to service.
In class: Instructor demonstrates disassembly and clearance measurement of an external gear pump, measuring gear-to-housing clearance and end-plate clearance with a feeler gauge; students record the procedure and specification limits. · Students disassemble a double-diaphragm pump, inspect diaphragms for cracking, delamination, or distortion, inspect ball valves for wear or debris, reassemble with new diaphragms, and document component condition on an inspection form. · Class-wide discussion and diagram exercise on relief valve sizing and placement in a positive-displacement pump circuit: students draw a schematic showing pump, relief valve, isolation valve, filter, and actuator, and explain why bypassing the relief valve would be a safety hazard.
Materials: External gear pump (training unit), double-diaphragm pump (training unit), replacement diaphragm kits, feeler gauge set, combination wrenches, manufacturer service manuals, schematic paper, inspection forms, lab notebooks, safety glasses, nitrile gloves
Check: Instructor inspects reassembled diaphragm pump for correct diaphragm seating and proper valve ball orientation; student must score 80% or higher on a 10-question exit quiz on PD pump principles and relief valve function.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

NCCER Industrial Maintenance — Hydraulics and Pneumatics
1. What fundamental characteristic distinguishes a positive-displacement pump from a centrifugal pump?
NCCER Industrial Maintenance — Pump Systems
2. During gear pump disassembly, a feeler gauge measurement of gear-to-housing clearance shows the 0.005 in blade passes with light drag and the 0.006 in blade will not enter. The manufacturer's maximum clearance is 0.004 in. What is the correct conclusion?
NCCER Industrial Maintenance — Pump Systems
3. When inspecting the diaphragms of a double-diaphragm pump, which condition requires immediate replacement of the diaphragm?
NCCER Industrial Maintenance — Hydraulics and Pneumatics
4. Where must a relief valve be positioned in a positive-displacement pump circuit to provide proper overpressure protection?
MSSC CPT — Safety and Quality
5. A technician caps (plugs) the relief valve outlet on a gear pump circuit during a test. What is the most likely outcome?
NCCER Industrial Maintenance — Pump Systems
6. What is the purpose of ball check valves in a double-diaphragm pump?
MSSC CPT — Safety and Quality
7. Before disassembling any pump for maintenance, which procedure must be completed first according to OSHA standards?
NCCER Industrial Maintenance — Pump Systems
8. A gear pump shows reduced output flow even though shaft speed has not changed. Which internal condition is the most likely cause?
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