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This article was first published in 2001.
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Coil springs are one of the most active components of a vehicle. They absorb major road irregularities and a constant barrage of small amplitude, high frequency "patter". Therefore - due to their relentless action - it's very important that any aftermarket springs you buy are formed using the very best techniques. This will maximise safety and protect against failure. K-Mac (Australia's longest established suspension specialist) has been using one of these techniques - resistance heating - for many years. Here we'll take you from start-to-finish through the company's high quality spring manufacturing process...
K-Mac employs a purpose-developed computer-controlled coil winder, which they originally developed back in 1978. The main body of the machine accommodates a slipover mandrel, onto which the spring wire is wound. There are over 70 different mandrels available to cater for each car's coil spring length, inner diameter and taper (if any). The system's PC stores all vital spring specifications for just about every car ranging from old Ford Customlines to the 2001 Chrysler PT Cruiser. The first step in the spring forming process is to key in the exact details of spring required. Variables such as linear or progressive rate and the desired ride height are customer-specified. The program will then calculate the exact wire diameter and length required to form the coil.
K-Mac uses BHP XK9261S steel to construct their coils. Its high chromium content is said to give prolonged resistance to spring sag. The computer-specified wire diameter is selected from a pile and then cut to length.
Cutting the wire to its correct length is very important. K-Mac achieves consistency with metal stoppers at set distances away from the blade of their heavy-duty cutting machine. The leading end of the wire is pushed against the appropriate stopper (which gives the correct distance back to the cutting blade), while the trailing length is lifted and chopped.
A furnace is the most common method of heating (and therefore softening) the wire before it's wound. However, K-Mac uses the far-superior resistance heating technique. This is where the cut-to-length wire is suspended between two electrodes. A full 600 amperes (at 415 volts) is then passed through the wire for anywhere between 25 seconds and 2 minutes (depending on wire thickness and length).
The resistance heating technique is said to have significant benefits over a conventional furnace. Unlike an oven-like warming process, resistance heating increases wire temperature from the core outward - giving a more uniform heat distribution through its cross-section. Traditionally, a wire is heated using a gas or oil fired oven, which blasts heat onto the spring surface. This process often leads to scaling and surface imperfections.
Once the wire has an evenly spread orange glow, it's soft enough to be wound onto the coil winding machine. The current supply is then switched off and one end of the wire is picked up with a pair of long handled tongs. Notice that the leading end of the wire must first be fist put into a small flame prior to coil winding. This softens the tip, which otherwise misses out on receiving as much heating current.
The still-glowing wire is then passed to the nearby coil-winding machine and wound onto the pre-selected mandrel. A switch is used to control the rotation of the mandrel, while the wire is simultaneously tongs-fed. The coil-winding machine typically takes around 15 seconds to form the coil.
Once formed, the coil is removed from the mandrel and placed on a stand for inspection. It is assessed for parallel sides, even coil spacing and correct wire termination positions at both ends. Those coils requiring further end tapering or a pigtail (that short length at the end of the coil that's flipped in towards the centre) are then put over another special mandrel. The wire is still hot enough to allow easy coil adjustment.
The next step is to quench the new coil in a bath of oil. Quenching serves to rapidly cool the wire, giving the optimum blend of strength and ductility. The coil is removed from the tank after a few minutes.
Again, K-Mac uses a modern approach when it comes to tempering. Instead of placing the spring into a kiln for an extended length of time, it is - once more - heated using electric current. This gives the benefit of uniform temperature distribution through the wire's cross-section. This re-heating burns off any residual oil and generates a finished hardness that will resist sagging, but still be ductile enough to prevent breakage.
Note that all automotive springs require 390-grade shot peening (although we didn't see it getting done). The aim here is to relieve stress and remove any scales that are left over after the resistance tempering procedure.
The next step is known as scragging. This is where the coil is compressed to its fullest position and - once released - its free-length is measured for consistency.
This process further prevents spring sag and creates a permanent free-length set.
At this point, a large percentage of coils need to have one (or both) of the ends ground flat. This needs to be performed whenever the end of an OE coil has been ground flat - the primarily function being to enable the spring to sit securely in its mounting. K-Mac uses a heavy duty grinding machine that progressively removes wire material from the end coil.
With the coil physically ready for use, the final step is to beatify the product. K-Mac - as with many of their products - powder-coat the coil orange. Powder coating gives more durability than a painted surface. Once dry, the distinctively coloured K-Mac coil spring is ready for packaging and making its way onto your car.
Interested in a pair?
Now that you know K-Mac make a pretty decent coil spring, you may be wondering about their pricing and availability. Uniquely in Australia, K-Mac can meet your specifications for ride height (usually to within a few millimetres) and your preference for linear or progressive rate coils. And - chances are - they can make a coil to suit any vehicle. We're told that they cover everything from Rolls Royce to Ferrari!
Pricing is fixed; European and 4WD springs cost $195 a pair, while those for a typical vehicle will set you back $165. Not bad considering you also get a 12-month unconditional warranty!
Contact:
K-Mac Head Office
+61 2 9556 1799
www.k-mac.com.au
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