We at Leaf Spring are constantly evolving and upgrading all our technology and keeping up with the latest process so that our unique “quick-change” processes and computerized production methods are the keys to a world-class operation which is specifically suited to meet the clients’ requirements and as O.E. Specifications.
On receiving the raw material, (Spring steel flats of 18 ft length) are stacked in the stockyard gradewise. Most commonly used grades are En45a, 60Si7, 65Si7, Sup9, Sup9a, Sup 11a etc. Cross Section of the steel flats ranges from 45x6mm to 120x40mm depending upon the specific customer requirements. Before processing, raw material is subjected to receiving inspection such as dimensional, chemical & metalurgical inspection. On approval it is sent for shearing operation.
In this operation, the leaves that constitute an assembly are cold sheared to the required lengths in the shearing machine based on the drawing provided by the customers.
After shearing, leaves of the Parabolic Springs are sent for parabolic profile rolling while those of Multi Leaf Springs are sent to drilling (As Multi Leaf springs do not involve parabolic profile rolling). In this operation one end of the leaf is heated in oil fired furnace. On achieving the desired rolling temperature (950 deg C) the leaf is put on to the conveyor by the walking beam furnace. The hot leaf then enters the parabolic rolling machine, where it is gripped at the cold end and rolled to the specific profile fed in the computer. The same is repeated on the other end.
In this operation, the leaves are drilled at the centre on all the leaves and at the ends on the specific leaves for bearing wear pads and clips depending upon the design of the springs.
After drilling, the main leaf or the first leaf of the assembly is sent for this operation. In this operation the leaf ends are heated in oil fired furnace to 950 deg C and rolled to the specific diameter for enclosing bushes for fitment.
After Eye Rolling, the eye diameter is machined to the final size by carbide or stainless steel reamers to encompass the bushes with the desired interference fit.
After drilling, the second leaf and the subsequent leaves are sent for end cropping where the leaves are heated at the ends in the oil fired furnace to 950 deg C to crop the leaves to the desired size based on the drawing.
After cropping, the cropped second leaf is reheated to form a wrap on one end or both the ends as desired to provide protection in service on failure of first leaf.
After all the end processing is over, the leaves are passed on to the heat treatment zone. In this the leaves are heated in the oil fired Walking Beam Furnace controlled by PID System to 920 Deg C. Depending upon the cross section of the leaf to be heat-treated, the stroke and cycle time are adjusted to ensure the total travel time in the furnace. On heating the structure of the steel is completely austenitic.
After the specific total travel time, the leaves drop out at the exit of the furnace and are picked and placed on the cambering tool where the desired bend is given to the individual leaves.
On providing the desired bend the cambering tool opens up, on which the leaf is picked up and quenched in the oil. The temperature of the oil tank is maintained between 70 deg C to 90 deg C to ensure correct rate of cooling. The austenitic structure changes to martensitic structure on quenching. It takes about 15 min for the leaf to come out of the quenching tank.
Leaf is then placed in the tempering furnace where the temperature is maintained at 480 deg C to 520 deg C depending upon the grade of the steel being used. It takes about 1 hr 45 min for the leaf to travel through this furnace. Leaf is exposed to the hot air draft. The micro structure is changed to tempered martensite. This operation imparts toughness to the steel by reducing the hardness. The temperature of the furnace is also controlled by PID System within ± 5 deg C.
Leaves coming out of the tempering furnace are tested for hardness on Brinell Hardness Testing Machine at periodic intervals to ensure that the desired hardness is achieved. The desired range of hardness is 388 to 444 BHN.
Leaves are then subjected to shot-peening where cut wire steel shots of 1 mm strike the tension surface to induce compressive stresses and thereby improve the fatigue life of the springs. Machine is set to the desired intensity by periodically calibrating it. In case of Parabolic Spring’s, every individual leaf is stress-peened in which every Parabolic Leaf is deflected to flat load & held in a tray & shot-peened under load.
Leaves are provided with the initial coat of zinc chrome primer by spraying a minimum of 35 microns of thickness.
Main leaf is fitted with the bushing as desired by hydraulic machine.
Main leaf ends are grinded to the specified width to ensure fitment. A double wheel grinding machine is used for this purpose.
Bush inner dia is reamed to the final size to ensure fitment of the spring eyes by means of carbide reamers mounted on the reaming machines.
Clips are riveted to the leaves by means of rivets. These clips prevent the fanning of the leaves in service.
Tension face of all the leaves are coated with graphite paint to reduce the interleaf friction in service. Graphite application is done by means of spray gun.
All the leaves are assembled on this machine and secured by means of centre bolt which passes through the centre of all the leaves. The desired torque is applied depending upon the size of the bolt.
The entire assembly is then subjected to a load by mounting the ends of the springs on the trolleys and applying the load at the centre by mean’s of an hydraulic machine. The load is equivalent to the 90% of the yield stress value. Scragging prevents the assembly from getting sagged in service and also relives the assembly from the internal stresses. Every assembly is subjected to scrag load 3 times in quick succession.
Spring’s are checked to ensure the parallelism of eyes with respect to seat to ensure easy fitment.
Clips are then folded on Clamp folding machine to prevent the fanning of the leaves in service.
Springs are then sent for painting where the entire assembly is painted by enamel or Nitro cellulose paint by spray guns in painting booths to prevent rust in service. The total coat thickness is approx 70 microns minimum.
Spring’s are marked by screening or stenciling for identification purpose.
Springs are then stacked on the pallets with each layer separated by wooden ribs and then covered by air tight covers and then secured by steel strips to the pallets for dispatch.