Spring Seats

A spring seat, also called a spring isolator or spring spacer, is located on the exterior of the shock absorber body or at the upper end of the piston rod. It positions and supports the suspension coil spring, normally in contact with the lower or the upper end of the spring inside the shock absorber assembly. It is made from NR, NBR, EPDM, PU, HDPE or TPE, with or without metal bonded to it.

The part carries the full spring load for the life of the assembly, so its performance depends on how the material is compounded, how the metal insert is prepared and how the cure cycle is controlled. This guide describes what a buyer should expect to see controlled during manufacture and which documents to request with each batch.

Reading the Drawing for Production

The drawing is the only document that ties the tool, the compound and the inspection plan together. The dimensions are the straightforward part. The features that decide whether the seat survives its design life are usually the ones that are easy to leave out.

Where the drawing is silent on the spring end form or the permitted compression set, the supplier will still produce a part with the right outline, but it may not hold its height or keep the spring located, and the difference appears in service rather than on the inspecting bench.

Compound Selection and Batch Control

Each material family brings its own process requirements, and a compound that is not controlled at the mixing stage cannot be corrected later in the press.

Compound traceability matters for a safety related position. A batch code that reaches back to a compound lot is what allows a field failure to be compared against the specification rather than simply replaced.

Metal Insert Preparation and Bonding

Where a metal insert is bonded into the seat, the bond is created during vulcanising from a primer and an adhesive applied to prepared metal. Four variables have to be controlled together.

1. Surface preparation. Degreasing followed by a phosphate or conversion coating or grit blasting, with parts handled in clean gloves afterwards, because fingerprints on a prepared surface are a recognised cause of local bond failure.
2. Film weight. Too thin and the bond is incomplete, too thick and the adhesive layer becomes a weak link in its own right.
3. Drying and open time. Bonding onto a wet or contaminated film produces a joint that passes the first inspection and fails in service.
4. Cure temperature and time. Under cure leaves the elastomer soft and the bond weak, and over cure hardens the compound and raises the stiffness.

For plastic seats in HDPE or TPE the insert is normally moulded in or assembled with a mechanical interlock rather than bonded, and the relevant control is the fit and the retention force rather than bond strength.

Moulding and Vulcanising

Compression moulding remains suitable for short runs and large seats, while transfer and injection moulding give better consistency at higher volumes. The controls that matter to the buyer are the same whichever route is used.

Trimming, Drilling and Hole Positioning

Where the mounting holes are formed after moulding rather than in the tool, the drilling jig and the trimming operation become part of the quality plan.

• Hole position tolerance should be stated on the drawing and checked against the jig, because a hole that is a few tenths out will still fit and will preload the seat on assembly.
• Trimmed edges should be free of tears and should not expose the metal insert, since an exposed edge invites corrosion.
• Where the hole breaks through into the spring contact area, the surface should be deburred so it does not wear the spring coating.

Dimensional and Performance Verification

Load, Compression Set and Durability Testing

A dimensional report confirms that the part resembles the drawing. A hardness, load deflection and compression set report confirms that it will keep the ride height. A bond result confirms that the insert will stay in place. For a seat under continuous spring load, all three groups are worth requesting.

In Process Controls and Traceability

1. Incoming inspection of compound, inserts and adhesive, with batch references retained.
2. Preparation line controls recording the sequence and the time between preparation and bonding.
3. Vulcanising records per cycle, with the actual temperature profile rather than the set point alone.
4. Batch identification on the part, the bag and the carton, so a complaint can be traced back to a mould, a press and a compound lot.
5. Retained samples from each batch under defined storage conditions, so that a future claim can be compared against a reference.

Quality management to IATF 16949 supports this route by requiring that dimensional tolerances, weld strength where applicable and coating adhesion are controlled and recorded rather than checked informally.

Packaging and Storage

Elastomer parts continue to age in storage, and a seat kept in sunlight, in ozone rich air or near an electric motor will degrade before it is fitted. Cartons should be closed and labelled with the part number, the batch code and the production date. Recommended storage is a cool, dry, dark place away from oils, solvents and ozone sources, with stock rotated on a production date basis. Parts with a bonded insert should be packed so that the insert edge cannot cut into the elastomer of the part next to it.

Supplier Evaluation Points

These points are worth confirming before a first order rather than after, because a seat that is correct on the drawing and wrong in the compound or the bond is difficult to detect at goods receiving. Customisation of diameters, heights and mounting hole positions is quoted from drawings or samples under custom moulded and bonded parts.

Frequently Asked Questions

Why does the same reference from two suppliers feel different on the vehicle?

Because the installed behaviour depends on the compound, the hardness and the insert design rather than on the outline alone. Two seats can share the diameter, the height and the hole pattern while using different materials or a different insert, and the difference shows up as ride height variation, noise or early compression set rather than as a visible defect.

What should be checked on a bonded insert?

The bond quality and the surface preparation, because a bonded insert carries the location function as well as part of the load. A peel or push out result for the production batch is the practical evidence, together with a visual check that no adhesion failure has started at the edge of the insert.

Does compression set matter for a spring seat?

Yes. A seat that takes a compression set allows the spring to sit lower, so the ride height changes even though nothing has broken. Requesting a compression set value at the design load is the way to confirm that the part will hold its height rather than simply surviving the load.

Does a change of tooling require requalification?

For a safety related suspension position, yes. A change of mould, press or preparation line can alter the cure profile and the bond quality without any change to the drawing, so a first article with dimensions, hardness, load deflection and bond results should be approved before full production resumes.

Manufacturing Support for Spring Seats

Bonpora Parts manufactures spring seats, spring isolators and spring spacers in NR, NBR, EPDM, PU, HDPE and TPE, with or without a bonded metal insert, moulded to the diameter, height, spring end location and mounting hole positions shown on your drawing. Incoming compound and insert controls, recorded vulcanising cycles, hardness and load deflection verification, compression set and bond testing and batch traceability are part of the standard route. Start with the spring seat range, read the fitting sequence on the spring seat installation guide, review the wider chassis and suspension programme, or contact our team with your specification and annual volume. Related shock absorber parts are supplied on the same route, and further references are listed on the products page.