Powertrain application
Oil pump
springs.
The compression springs that set the oil pressure and control the flow in engine oil pumps and hydraulic pumps — wound to your drawing in Rayong, Thailand from 0.08 mm to 8.0 mm wire, with load and fatigue testing in-house.
A small spring that decides the oil pressure
Inside an engine oil pump there is a relief valve, and the spring behind it is what sets the pressure the engine actually sees. Oil pressure pushes on the valve face; when that force exceeds the spring force, the valve lifts and oil bypasses back to the sump.
The spring strokes only a few millimetres, usually 1 to 3 mm, but it is loaded continuously whenever the engine runs. The difficult requirement is rarely the stroke. It is load stability: a spring that relaxes by 8% at operating temperature delivers 8% less oil pressure at idle for the rest of the engine’s life, and no amount of tightening on the bench brings it back.
In a variable-displacement vane pump the control spring does the opposite job. It holds the cam ring against the control piston until pressure is high enough to move it, so the spring defines where the pump starts to destroke — and therefore the flow curve, not just the peak pressure.
- Wire diameters from 0.08 mm to 8.0 mm in current production
- Straight, conical and variable-pitch windings
- Closed and ground, closed or open ends as drawn
- In-house stress relief and tempering
- Load measured at installed length and at valve lift
- Prototype, pre-production and volume orders
Where the springs sit
The relief spring sits in a bore in the pump body or cover, behind the relief plunger, ball or spool. One end bears on the valve, the other on a seat, a plug or an adjusting shim. The installed length is set by that stack, so the spring’s free length and rate are what translate a seat diameter into a cracking pressure.
In a variable-displacement pump the control spring sits in the cam ring bore, opposed by the control piston. A third family — the plunger return and check valve springs — sits in the suction and delivery passages and works against oil flow rather than against a regulated pressure.
The numbers that define a pump spring
Six values decide the coil. If the drawing carries them, the rest of the specification can be confirmed from the geometry.
Cracking pressure
The pressure at which the valve first lifts. Together with the seat area it fixes the preload the spring must apply when the valve is closed.
Effective seat area
The bore the oil pressure acts on, usually the valve seat or spool land diameter. Force equals pressure times this area — it is the conversion between bar and newtons.
Installed length
The length the spring is held at with the valve seated, set by the bore depth and the plug or shim stack. Every load figure depends on it.
Lift and full-flow pressure
The stroke the valve opens and the pressure at that lift. Seat point and full-flow point together fix the spring rate.
Oil temperature range
Sump oil commonly runs between 100 °C and 150 °C. Sustained temperature decides the grade and the allowance for load loss over life, not the finish.
Pulsation and cycle target
A relief valve chatters as it relieves. The spring sees a large number of small cycles on top of the pressure cycles — the test plan has to cover both.
From cracking pressure to spring load
Pressure becomes force through the seat area. Everything else on the drawing follows from that force and the two lengths it acts over.
A relief valve with a 12 mm seat has a pressure area of π/4 × 12² = 113.1 mm². Cracking at 4 bar (0.4 MPa) needs 0.4 × 113.1 ≈ 45 N of spring force. Fully open at 6 bar (0.6 MPa) needs 0.6 × 113.1 ≈ 68 N. If that lift is 2.5 mm, the rate is (68 − 45) ÷ 2.5 ≈ 9.2 N/mm.
The rest is arithmetic and margin. At a 24 mm installed length the spring is deflected 45 ÷ 9.2 ≈ 4.9 mm, so the free length is about 28.9 mm before allowance for set. At full lift the length is 21.5 mm, so solid height has to sit below that with clearance to spare. And because the valve also sees flow forces and spring-chamber pressure, the calculation is a starting point that the pump bench confirms.
What has to agree
- Seat diameter against cracking pressure
- Installed length against preload
- Lift against full-flow pressure
- Rate calculated from both load points
- Solid height against full lift with clearance
- Load at operating temperature, with relaxation allowance
- Stress range against the cycle target
Relief, control, return and check
Most pump spring drawings we quote fall into one of these families. All are compression springs; what changes is the load regime and the space the pump gives them.
Straight winding
The standard arrangement: one helical compression spring behind a plunger, ball or spool, set to a cracking pressure by its installed length. The most common spring in an oil pump, and the one the pressure specification is written around.
Rising rate in a short bore
A conical spring nests as it compresses and gives a rising rate in a shorter installed length, with a natural frequency that moves away from the valve’s chatter. Used where the cover depth is tight or the valve hunts.
Variable displacement
Holds the cam ring of a variable vane pump until the control piston overcomes it. Sets where the pump begins to destroke, so its load tolerance shapes the flow curve rather than a single pressure point.
Suction and delivery
Returns a plunger or piston against oil flow rather than a regulated pressure. Usually lower stress, but it has to survive contamination and the full oil temperature range.
Delivery and anti-drain-back
Small, light springs that hold a ball or disc closed until flow opens it. Low load, high cycle count, and sensitive to the seat geometry around them.
Filter and cooler bypass
Opens when a filter blocks or the oil is cold and viscous, protecting the circuit from pressure spikes. The setting is a differential, so the spring is specified against a flow condition rather than a fixed pressure.
All six are wound to the drawing. Where the design calls for a damper, a coating on a nested spring or a process outside our in-house list, it is reviewed and agreed at RFQ rather than assumed.
Oil pump spring specifications
Current production is shown below. Parameters outside it are reviewed at RFQ rather than assumed.
| Parameter | Capability |
|---|---|
| Wire diameter | 0.08–8.0 mm in current production. Planned expansion to 12.0 mm as new equipment is installed. |
| Materials | 65Mn, 72A, 82B and 60Si2Mn carbon spring steels; stainless steel 301, 302, 304 and 316. Other grades reviewed on request. |
| End form | Closed and ground ends, closed ends or open ends, as defined on the drawing. |
| Installed length and preload | Measured at the installed length stated on the drawing; the load tolerance is agreed at RFQ. |
| Load at lift | Measured at the valve lift stated on the drawing; the rate is confirmed from the two measured load points. |
| Load stability | Stress relief and tempering in-house. The allowable load loss over service life is agreed at RFQ against the oil temperature range. |
| Heat treatment | In-house stress relief and tempering. |
| Surface finish | Anti-rust oil, electrophoresis (E-coat), zinc plating, nickel plating, spray coating; passivation for stainless steel springs. |
| Wire tolerance | JIS wire tolerance by grade (JIS G 3521, JIS G 3522 or JIS G 4309). ASTM wire specification and SMI commercial tolerances available for US programs. |
| Dimensional inspection | 2.5D projector measurement of coil geometry and end squareness, plus dimensional checks against the drawing. |
| Load testing | Automatic spring testers; load at installed length and at lift recorded. |
| Fatigue testing | Cycle-life testing in-house, to the load range and cycle count agreed at RFQ. |
| Material testing | Raw material tensile strength and metallographic structure. |
| Corrosion testing | Salt spray testing for plated and coated parts. |
| Quality system | ISO 9001:2015 and ISO 14001:2015. Automotive quality management certification in progress. |
| Samples | Prototype and pre-production samples against drawing, with the inspection record agreed at RFQ. |
| Packing | Bulk, layer-packed or customer-specified packaging agreed at RFQ. |
Please state the seat diameter, the cracking pressure, the full-flow pressure and lift, the installed length and the oil temperature range. The first four decide the wire and the rate; the temperature decides the grade.
Which wire grade for which duty
Grade follows stress, temperature and the corrosion protection the drawing calls for. The table below is a starting point, not a substitute for the pump specification.
| Grade | Type | Typical use in a pump |
|---|---|---|
| 65Mn | Manganese spring steel | General pump and valve duty where hardenability and cost matter more than the highest fatigue strength. |
| 72A | High-carbon spring wire | Moderate-stress springs in volume production; a cost-efficient baseline grade. |
| 82B | High-carbon spring wire, higher carbon | Higher tensile strength for higher working stress in the same bore. |
| 60Si2Mn | Silicon-manganese spring steel | Higher-stress duty and sustained oil temperatures, where resistance to load loss matters. The grade normally specified where the spring runs hot for its whole life. |
| 301 / 302 | Austenitic stainless, spring temper | Corrosion resistance without a coating, for hydraulic circuits or wash-down environments. |
| 304 | Austenitic stainless | General corrosion resistance for springs that see moisture or condensation. |
| 316 | Austenitic stainless with molybdenum | Chloride-bearing or more aggressive fluids where 304 is not enough. |
Load per millimetre
65Mn, 72A, 82B and 60Si2Mn carry the highest load in the smallest wire, which is what a shallow pump cover usually needs.
Corrosion without coating
301, 302, 304 and 316 remove the dependency on a plated barrier, at a lower modulus and a different fatigue profile.
Oil · E-coat · zinc · nickel · spray
Anti-rust oil, electrophoresis, zinc plating, nickel plating and spray coating are available. The finish is chosen for the fluid and the assembly process.
From wire to tested pump spring
Every stage below runs in the Rayong plant. The sequence matters: stress relief before grinding, tempering after forming, inspection after finishing.
Incoming wire
Diameter, tensile strength and metallographic structure checked before the wire is released to production.
Coiling
CNC spring forming to the drawing: wire diameter, mean coil diameter, active coils, pitch and end form.
Stress relief
Low-temperature treatment removes the forming stress before the ends are ground, so the spring does not move afterwards.
End grinding
Closed ends ground square to the axis to give a stable seat and a defined installed length.
Heat treatment
Tempering to the hardness and strength the grade and the stress level require.
Surface finish
Anti-rust oil, E-coat, zinc, nickel or spray coating; passivation for stainless steel springs.
Inspection
Load at installed length and at lift, coil geometry on the 2.5D projector, and the test plan agreed at RFQ.
Packing
Layer-packed or bulk, with the lot identification the customer’s incoming inspection requires.
Records
Inspection results retained and issued with the shipment in the form agreed at RFQ.
Load and life, measured before shipment
A relief spring that is right on the first part and wrong on the ten-thousandth gives a pump that holds pressure on the bench and not in service. Load, dimensions, material condition and cycle life are checked in-house.
- Raw material tensile strength and metallographic structure
- Load testing at installed length and at valve lift
- 2.5D vision measurement of coil geometry and end squareness
- Fatigue testing for cycle-life validation
- Salt spray testing for plated and coated parts
- Inspection records agreed at RFQ
Failure modes a pump spring is designed against
Pump springs rarely fail because the wire was the wrong diameter. They fail because one of the mechanisms below was not in the drawing.
Pressure that drifts down
A spring held hot under load loses force over time. Grade, stress level and oil temperature together decide how much; the drawing should state the acceptable loss at the end of life.
Cycles on top of cycles
A relief valve opens and closes rapidly while relieving. That chatter adds small cycles at the highest stress point, and the coil inner radius is where they show first.
Resonance in the spring
At some flow and pressure combination the spring’s natural frequency matches the valve’s oscillation. The valve hammers, pressure oscillates and the spring fatigues quickly.
Sludge and metal particles
Particles lodge between coils and hold the spring partly compressed, or jam the plunger. A spring that cannot return fully is a spring that cannot hold its pressure.
Water and acid in the oil
Condensation and combustion by-products reach the pump. Plated parts rely on the barrier staying intact; stainless removes the dependency but changes the modulus.
Contact at the ends
The ground end faces bear on the valve and on the plug. Squareness, surface finish and the hardness of the mating parts decide whether the spring wears or the seat does.
What to send with your drawing
Pump spring quotes go wrong when the cracking pressure is quoted without the seat diameter. Send both, and the rest follows.
- Seat or spool diameter the pressure acts on
- Cracking pressure, with the unit used (bar or MPa)
- Full-flow pressure and valve lift
- Installed length, and free length if it is fixed
- Maximum outside diameter and bore depth
- Oil type and operating temperature range
- Cycle-life target and any pulsation requirement
- Material, finish and documentation requirements
- Annual quantity and delivery schedule
PDF, DXF, STEP or a clear photo
Email your drawing to info@aofengspringthailand.com. We confirm the wire, the coil design, the material and the test plan before quoting.
If the drawing is incomplete, send what you have and the pump details — type, displacement, pressure range and oil temperature. We will list the open points rather than guess them.
Oil pump spring questions
What is an oil pump spring?
It is the compression spring that sets the pressure in an engine oil pump or hydraulic pump. In a relief valve it holds the valve closed until oil pressure overcomes the spring, then lets excess oil bypass. In a variable-displacement pump a control spring sets where the pump begins to destroke.
How do you size a relief valve spring?
Convert the cracking pressure into force using the seat area — pressure times area — then do the same at the full-flow pressure and lift. The two forces and the two lengths give the spring rate. The installed length, solid height and operating temperature then confirm the wire and the grade.
What do you need to quote an oil pump spring?
Seat diameter, cracking pressure, full-flow pressure and lift, installed length, maximum outside diameter, oil temperature range, cycle-life target, material, finish and annual volume. With those we can confirm the coil design and the test plan.
Which material do you recommend for hot oil?
Where the spring runs at sustained oil temperature, 60Si2Mn is normally specified because silicon-manganese spring steel resists load loss better than plain high-carbon wire. 65Mn, 72A and 82B suit cooler or lower-stress duty, and stainless grades are used where corrosion protection without a coating is required.
Can you wind conical and variable-pitch springs?
Yes. Straight, conical and variable-pitch compression springs are wound to drawing, including nested outer and inner pairs where the pump design calls for them.
What wire diameter range can you wind?
0.08 mm to 8.0 mm in current production. Expansion to 12.0 mm is planned as new equipment is installed; parts above the current range are reviewed case by case.
Can you fatigue test pump springs?
Yes. Fatigue testing is carried out in-house to the load range and cycle count agreed at RFQ, and the load at installed length and at lift is measured on automatic spring testers.
Are you IATF 16949 certified?
Not yet. The Rayong plant is certified to ISO 9001:2015 and ISO 14001:2015, and certification of an automotive quality management system is in progress. We state this plainly so that sourcing decisions are made on the current position.
Send a drawing, get a quote
Tell us the seat diameter, the cracking pressure and the oil temperature. We will confirm the wire, the coil design and the test plan before quoting.
AOFENG SPRING (THAILAND) · Rayong, Thailand