There's a lot that can be done to ensure maximum performance

Have you ever wondered how many springs there are in the average family car? Hundreds. And many of them are made by the William Hughes Group.

William Hughes has been involved in the manufacture of custom-made springs and wire forms for over 225 years. Some say the Company’s first involvement with modes of transport began with making springs for stagecoaches.   Today, William Hughes has factories in Dorset, Bulgaria and Poland to cope with the demand for its high precision products.

Springs large and small roll off the production line in quantities large and small.  Compression, tension and torsion springs, all made to order, all solving a particular problem for customers.

Let’s trace the journey of a small red 003 CTS compression spring commissioned by a major motor manufacturer needing to improve the yield from their existing springs, which was running at about 80%.    Made of spring steel, this particular spring forms part of the accelerator mechanism of a car and is known as a ‘kickdown’ piston spring.

The William Hughes engineering team set to work to figure out how the Company’s manufacturing processes could be employed to provide the necessary upgrade.   Prototype springs were submitted for evaluation before being approved for bulk production.

The first process in the manufacture of the upgraded spring was coil winding. 

Explains William Hughes’ Engineering Director, Emma Burgon: “Normally for our customers, the most important thing is load and a specific length. The loads on a spring, that is, the force that it gives at a specific length, are directly proportional to the amount of wire in the spring.  This ‘load at length’ is what’s important to the customer.    We use the latest CNC technology not only to provide very consistent coiling accuracy but also to measure the free length of every single spring as it comes off the machine.  The machine then uses the statistical process control to sort every single part, so we know that is what is coming off the machine is good at this point to provide the tight tolerances demanded by our customers”.

Any under-length springs are scrapped while the good springs – normally around 98% of the batch – are stress relieved by passing them through an oven at typically 250ºC.

Each spring is then ground flat at both ends using our bowl-fed automated Bennet Mahler grinding machine.

The springs must achieve 1 million cycles from pre-load so to extend their working life they are next subjected to a glass shot peening process.

Burgon continues, “Shot peening is a mechanical process that de-stresses the surface of the spring, reducing the likelihood of fractures due to the formation of cracks.   It creates compressive stresses which makes it harder for cracks to propagate.   The surface of the spring is smoothed to obviate nicks etc.   It is important that the shot peening gets to the inside of the spring.   In the shot peening process, glass beads are thrown at the springs with great force which has the effect of welding together imperfections on the surface, significantly increasing fatigue strength and extending the service life of the spring”.

Next the springs are put into a basket and dipped in a tank of a protective finishing agent called DeltaTone.  This inorganic finishing treatment provides the springs with a highly flexible corrosion resistant coat which is coloured red for identification purposes.   The springs are given two coats of DeltaTone and are cooked both in between each coat and afterwards for 15 minutes at 200ºC.

Finally, as part of the process to ensure the springs meet the tight tolerances required, they are pre-stressed by feeding them into an automated line that first compresses the spring to near solid and then load tests each and every one.  The customer requires the springs to be tested through 1 million cycles but William Hughes tests them through 10 million – just to be on the safe side!

The pre-stressing increases the fatigue life, and the sorting ensures that the customer has parts in a very tight tolerance band. By reducing the load tolerance, the customer can dramatically increase the yield on his production line. So, a little more spent on the spring makes a huge amount of sense.  

Let’s let Burgon have the last word: “This is an excellent example of how upgrading the specification of a relatively simple spring has provided significant benefits.  As a result of upgrading the spring in the accelerator assembly, yield has improved from 80 to 97%, saving considerable time and money for our customer”.