What Is a Metering Cartridge? Function, Types & Applications
In a centralized oil lubrication system, the pump pressurises the supply line and oil flows through the distribution tubing to each lubrication point. But how does each point receive exactly the right amount of oil — and not more or less? That is the function of the metering cartridge.
A metering cartridge is a precision valve device installed at each lubrication point — either mounted directly on the machine housing or on a manifold block. Every time the pump cycles, each cartridge opens, ejects a fixed preset volume of oil, and closes again. The result is consistent, precise delivery to every lubrication point in the system, cycle after cycle, with no variation.
How a Metering Cartridge Works
Inside the cartridge is a small piston and a spring-loaded check valve. When the central pump pressurises the supply line, the oil pressure acts on the piston, compressing the spring and pushing the piston forward. As the piston moves forward, it ejects a fixed volume of oil through the outlet to the lubrication point. The volume ejected is determined by the piston stroke length — fixed at manufacture.
When the pump cycle ends and supply line pressure drops, the spring pushes the piston back to its rest position. This draws the next dose of oil from the supply line into the cartridge chamber — ready for the next cycle. The cartridge outlet has a check valve that prevents oil from flowing back into the cartridge from the lubrication point side.
This principle — often called a single-shot injector — has several important properties:
- Volume is pressure-independent. The dose per cycle is determined by piston stroke, not by supply pressure. A cartridge at the end of a long supply line, which sees lower pressure than one near the pump, delivers the same volume as the cartridge nearest the pump.
- Consistent delivery regardless of back-pressure. If one lubrication point develops partial back-pressure (e.g., a slightly clogged grease fitting), the cartridge still ejects its full dose — it does not reduce delivery as a simple orifice would.
- Independent operation. Each cartridge operates independently. A problem at one lubrication point does not affect delivery to any other point in the system.
SP Engineers Metering Cartridge Series
SP Engineers manufactures metering cartridges in three series — D1, D2, and D3 — covering a range of oil volumes per cycle:
| Series | Typical Application | Compatible Manifolds |
|---|---|---|
| D1 | Light-duty lubrication points, small guideway sections, bearings with low oil consumption | M-1 to M-8 |
| D2 | General-purpose machine tool lubrication — guideways, ball screws, headstock bearings | M-1 to M-8 |
| D3 | Heavier lubrication points requiring larger oil volumes per cycle | M-1A to M-4A |
The series selection is made during system design based on each lubrication point's oil requirement per cycle — which depends on the bearing or guideway type, load, speed, and lubrication interval. SP Engineers specifies the correct cartridge series for each point when designing a centralized oil lubrication system.
Manifold Mounting
Metering cartridges are most commonly installed on manifold blocks rather than individually at each lubrication point. A manifold is a machined block with a single oil inlet and multiple outlets, each fitted with one metering cartridge. The manifold connects to the supply line at one point and distributes oil to multiple adjacent lubrication points through individual outlet lines.
SP Engineers manifolds for D1 and D2 cartridges are available as models M-1 through M-8 — serving 1 to 8 lubrication points from a single supply connection. Manifolds for D3 cartridges (M-1A to M-4A) serve 1 to 4 points. A typical CNC machining centre might use three or four manifolds — one for the X-axis guideway, one for the Y-axis, one for the Z-axis, and one for the ball screws — serving a total of 8–16 lubrication points from a single motorised lubrication unit.
Where Metering Cartridges Are Used
CNC Machine Tools
The most common application. Every modern CNC machining centre, CNC lathe, VMC, and HMC uses metering cartridges to deliver measured oil to guideway surfaces and ball screw nuts. The cartridges are mounted on manifolds in the machine column or saddle — close to the lubrication points they serve — and connected to a motorised lubrication unit via a main supply line.
Conventional Machine Tools
Lathes, milling machines, and grinding machines with automatic lubrication use metering cartridges in the same way. Many older machines originally fitted with manual oil cups or wick oilers are retrofitted with a motorised unit and manifold-mounted cartridges to eliminate manual lubrication.
Precision Machinery
EDM machines, jig boring machines, and gear hobbing machines all use metering cartridges where precise, repeatable oil dosing is critical for machining accuracy. Inconsistent lubrication on precision machines causes stick-slip on guideways, which translates directly to dimensional error in the workpiece.
Printing and Packaging Machinery
High-speed printing presses and packaging machines use metering cartridges for cam followers, eccentric bearings, and guide bushings that require consistent oil at short intervals. The alternative — manual oil cups — requires daily attention and is invariably neglected.
How to Know If a Metering Cartridge Is Working
The most common failure mode is blockage — either a blocked cartridge outlet or a blocked outlet line to the lubrication point. A blocked cartridge can be identified by removing the outlet line fitting and running a pump cycle: if no oil emerges from the cartridge outlet, the cartridge is blocked and should be replaced. Blocked cartridges are usually caused by contaminated oil — which is why in-line filtration on the supply side of the lubrication unit is important.
Some manifold designs include a check indicator — a small visible indicator that confirms oil flow through each outlet. These are useful for maintenance inspection without dismantling any connections.
Metering Cartridge vs Plunger Pump — Which to Choose?
Both devices meter oil to individual lubrication points, but they operate at different pressures and suit different applications:
| Parameter | Metering Cartridge | Plunger Pump |
|---|---|---|
| Operating principle | Passive — responds to system pressure from central pump | Active — generates its own delivery pressure |
| Delivery pressure | Depends on central pump (typically 10–30 bar) | Up to 200 kg/cm² independently |
| Typical application | CNC guideways, machine tool bearings — compact machine footprint | Longer line runs, high-resistance points, high-pressure requirements |
| Installation | On manifold block, close to lubrication points | Directly at lubrication point or on manifold, M22 × 1.5 thread |
| Complexity | Simple — no moving parts in service | Requires pump element maintenance over time |
Frequently Asked Questions
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