Positive Displacement Pump
A positive displacement pump moves fluid by trapping a fixed volume in a chamber and mechanically displacing it into the discharge line. Flow per cycle is essentially fixed, so output is set by pump speed rather than discharge pressure. Common types include reciprocating (piston, plunger, diaphragm) and rotary (gear, lobe, vane, screw, progressive cavity, peristaltic).
In a CNC manufacturing environment, positive displacement pumps are critical where repeatable metering or high-pressure, low-flow delivery is required. For high-pressure through-tool coolant, a PD pump holds stable pressure to the cutting zone during deep-hole drilling or small-orifice operations—pressure is created by tool resistance, and excess flow is bypassed through a regulating valve back to the reservoir. In lubrication systems, volumetric distributors meter precise oil quantities to slides, ballscrews, linear guides, and spindles, ensuring consistent lubrication instead of splash-fed uncertainty. For transferring viscous fluids like coatings or adhesives in finishing lines, PD pumps maintain flow with less sensitivity to viscosity changes than centrifugal pumps. A key practical implication is that a PD pump is a constant-flow machine; if discharge is blocked, pressure rises until a relief device opens or a line fails. This requires an external or internal relief valve on coolant manifolds, glue supply circuits, and oil headers to prevent overpressure from closed valves, clogged filters, or pinched hoses.
Why does a PD pump maintain flow when pressure increases?
Because it displaces a fixed volume per cycle; pressure is not the control variable—resistance is. The pump continues to deliver the same volume per revolution or stroke regardless of backpressure, up to the design limit.
Why are PD pumps preferred for high-pressure CNC coolant?
They can generate stable pressure at low flow for small tool orifices and deep-hole drilling, where centrifugal pumps are better suited to flood cooling. The positive displacement action ensures consistent coolant delivery even under varying system resistance.
Why must a relief valve be installed?
To prevent overpressure when discharge flow is blocked or the circuit restriction increases unexpectedly. Without a relief valve, the pump will continue to force volume into a closed system, causing component damage or catastrophic failure.