Bushing

A bushing, also called a suspension bushing or control arm bushing, is installed at suspension control arms, stabilizer bar connection points, subframe to body attachment points and similar locations, positioned between two metal components. It provides an elastic connection that allows relative motion, whether articulation or torsion, while absorbing road impacts and limiting radial and axial displacement of the suspension kinematic pairs. The tubes are normally carbon steel or aluminium alloy, and the elastomer is natural rubber (NR) or polyurethane (PU).

Two bushes can share a drawing and still behave differently, because in a bonded bush the performance comes from the metal preparation, the adhesive system, the compound and the cure cycle working together. This guide sets out what a buyer should expect to see controlled during manufacture and what to ask for in the inspection documents.

Reading the Drawing Before Production Starts

A bonded bush is only as good as the information on the drawing. The dimensions are the easy part. The features that decide durability are the ones that are often left unstated.

Where the drawing is silent on surface preparation, bond strength or stiffness, the supplier will still produce a bush, but it will be a dimensional copy rather than a performance copy, and the difference shows up in service rather than on the incoming inspection bench.

Tube Preparation and Metal Treatment

Steel tubes

Carbon steel shells are normally formed from tube or strip, cut to length, deburred and then prepared for bonding. The preparation sequence matters as much as the chemistry. Degreasing removes drawing and cutting oils, a phosphate or conversion coating gives the adhesive a stable surface, and the parts are handled with clean gloves afterwards because fingerprints on a prepared surface are a recognised cause of local bond failure.

Aluminium tubes

Aluminium is chosen where weight matters, typically on the outer shell of a large subframe bush or where a light alloy housing would otherwise suffer galvanic attack against a steel shell. Aluminium needs its own preparation route, because the natural oxide layer forms within minutes of cleaning and must be removed or converted immediately before bonding. Anodising is not a substitute for a bonding conversion coating; it improves corrosion performance but can make adhesion more difficult if it is not specified for that purpose.

Coating after bonding

Whether the shell is coated before or after vulcanising is a tool design decision. Coating before bonding requires the coating to tolerate the cure temperature and to stay clear of the bond area. Coating after bonding gives more freedom on colour and finish but requires masking so that the elastomer is not damaged by the coating process.

The Adhesive System and the Cure Cycle

A bonded rubber to metal bush is held together by a primer and an adhesive applied to the prepared metal, which co cure with the rubber during vulcanising. Three variables have to be controlled together.

• Film weight. Too thin and the bond is incomplete. Too thick and the adhesive layer becomes a weak link in its own right.
• Drying and open time. Bonding onto a wet or contaminated film produces a bond that passes the first test and fails in service.
• Cure temperature and time. Under cure leaves the elastomer soft and the bond weak. Over cure hardens the compound, raises the stiffness and can embrittle the surface.

These are process parameters rather than design parameters, which is why a change of moulding line or a change of press is a legitimate reason to ask for a requalification sample before the next shipment.

Compound Development

Natural rubber grades

Natural rubber remains the default for comfort oriented positions because it isolates vibration better than any comparable synthetic elastomer and offers good fatigue resistance at moderate cost. Its weakness is environmental: it needs an antiozonant and antioxidant package to survive sunlight and heat, and it swells in oil unless a resistant grade is chosen.

Polyurethane grades

Polyurethane offers higher load capacity for a given size, very good abrasion resistance and good oil resistance, which is why it is common on performance, heavy duty and off road applications. The trade off is isolation: a polyurethane bush transmits more road noise into the body, and the compound is more sensitive to moisture during processing.

Special formulations

Where the location is exposed to oil, diesel, cleaning chemicals or sustained low temperature, the compound family rather than the hardness has to change. The table below summarises the formulation directions that are normally available.

Moulding and Vulcanising

Compression moulding is the traditional route and remains suitable for short runs and large bushes. Transfer and injection moulding give better dimensional consistency and shorter cycles on high volume references. Whichever route is used, the controls that matter to the buyer are the same.

1. The preform weight is controlled so that the cavity fills without under packing or excessive flash.
2. Metal inserts are loaded into the correct location and held square, because a tilted insert produces a bush with an uneven wall on one side.
3. Vulcanising temperature and time are recorded per cycle rather than assumed from the set point.
4. Deburring and deflashing are done without cutting into the elastomer or nicking the shell edge.
5. The batch code is applied before the parts are packed, so that a mixed batch cannot leave the factory unidentified.

Dimensional and Stiffness Verification

Bond and Durability Testing

A buyer who receives only a dimensional report has confirmation that the part resembles the drawing. A buyer who also receives hardness, stiffness and bond results has confirmation that it will behave like the sample that was approved.

In Process Controls and Traceability

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

Packaging and Shelf Life

Elastomer parts age in storage, and a bush stored in sunlight, in ozone rich air or near an electric motor will degrade before it is ever fitted. Cartons should be closed and labelled with the part number, the batch code and the production date, and rubber parts should be packed so that the shells are not damaged in transit. Recommended storage is a cool, dry, dark place away from oils, solvents and equipment that generates ozone, with stock rotated on a production date basis rather than by carton position.

Supplier Evaluation Points

These points are worth confirming before a first purchase order rather than after, because a bush that is correct on the drawing and wrong in the bond is almost impossible to detect on a receiving inspection. New dimensions are quoted from drawings or samples under custom moulded and bonded parts.

Frequently Asked Questions

Why do two bushes that measure the same behave differently?

Because the installed behaviour comes from the elastomer compound, the void layout and the adhesive bond rather than from the dimensions alone. Two parts can share a bore, a shell diameter and a length while using different compounds, different hardnesses or different void areas, and the stiffness difference between them will be obvious on the vehicle.

Is a harder compound a direct upgrade?

No. A harder grade reduces deflection and sharpens the response, but it also passes more road noise into the body and increases the load on the surrounding mounts. Where comfort is the main complaint, a softer grade or a revised void layout usually helps more than a harder compound.

What documents should come with a production batch?

At a minimum, a certificate of conformance, a dimensional report and a hardness result for the batch, plus the batch code marked on the part and the carton. Where the application is safety related or the part has failed before, a bond test result and a stiffness result should be requested as well.

Does changing the moulding line require requalification?

Yes, on a safety related position. A change of press, mould or preparation line can alter the cure profile and the bond quality without changing the drawing, so a first article with dimensions, hardness and bond results should be approved before full production resumes.

Manufacturing Partnership for Bonded Bushes

Bonpora Parts manufactures suspension bushes and control arm bushes with carbon steel or aluminium tubes and natural rubber or polyurethane elastomer, moulded and bonded to the bore, shell, length, flange and void layout shown on your drawing. Incoming metal and compound controls, recorded vulcanising cycles, hardness and stiffness verification, bond testing and batch traceability are part of the standard route, and special formulations for oil, heat or low temperature service are available on request. Start with the mounting bush range, read how failed parts are diagnosed on the bushing failure analysis page, follow the fitting sequence in the bushing installation guide, or contact our team with your specification and annual volume. The wider chassis and suspension programme and the products index list the other bonded rubber parts we supply.