VOL. 18 March ISSUE YEAR 2017
MFN Trainer Column
in Vol. 18 - March Issue - Year 2017
The most important facts have already been mentioned in a study “SHOT PEENING IN THE DESIGN OF GEARS” by J.C. Straub, Chief Research Engineer of the American Wheelabrator & Equipment Corporation, Mishawaka, Indiana. This study was presented during the annual meeting of the American Gear Manufacturers Association 1953.
A very short extract from this presentation follows:
"Probably one of the most attractive applications for shot peening in design is that in which machine parts can be made smaller and lighter for more effective utilization of material. This application is usually associated with parts made in high-volume production. The incentive in this case is the decrease in cost of production without sacrificing quality."
Nowadays, new and further developments can be based on this knowledge, and I would like to give an introduction to the progress made in the production of cast crank shafts and piston rods as I have experienced them.
In the past, both parts were only forged and not cast. In the early seventies, foundries in Europe intensified development of suitable alloys that - in combination with shot peening - resulted in enhanced part properties so that crank shafts and piston rods could be cast.
I have had the opportunity and the privilege to work in a team that was responsible for this project and corresponding testing in the shot blast laboratory.
We started with parts for Italian and German car manufacturers.
The shot blast machines were a hanger type machine with two wheels and a special roller type machine suitable for laboratory purposes only.
For both machines and for both - crankshafts and piston rods - specially designed, tailor-made workpiece holding devices had to be made.
Test procedures on the hanger-type machine required repeated manual turning of the workpieces between different blasting cycles in order to fully expose critical areas to the blast stream at appropriate blasting angles.
The laboratory shot blast machine was equipped with two rollers on which a cage was arranged and put in rotation during operation. The workpieces had to be put individually into the cage. During blasting, the cage was moved forward and backward axially. Since the machine was only equipped with one blast wheel, the test parts had to be turned manually after half of the blasting time thus making sure that crank shafts or piston rods were shot-peened consistently.
In addition, different part series were shot-peened with different parameters (shot-size and shot-hardness, throwing speed etc.)
Time required for shot peening trials was between two to four weeks, depending on the different test procedures and the number of parts per series.
Tested and checked castings were then sent to the car manufacturers, where crankshafts and piston rods were integrated into engines of test vehicles. Extensive field trials and dismantling of test specimens for final checking of various part properties followed. Depending on the results, additional shot peening tests had to be performed in the laboratory.
Unfortunately, shot peening in batch mode cannot offer 100% process reliability. In the filled trough of a tumble blast machine, parts overlap so that they are not exposed to the blast stream and an inadequate or even a missing shot peening effect occurs.
This problem was addressed and attempts to resolve it were made by extension of the blasting time. However, the dwell time cannot be increased infinitely; otherwise, the markings on the castings are removed and are no longer legible. In addition, parts can be damaged or points of impact may occur during loading / unloading or during tumbling.
Appropriate actions have been taken and development work on improved machinery systems has been intensified.
Today, crankshafts shafts are shot-peened in a 100% process-safe manner using Manipulator machines and Rotary Chamber Blast Machines. Both machine types operate in fully automatic mode and are loaded or unloaded by robots.
The individual machine type is selected in accordance with production output (number of parts, length, weight and shape). Throughput is approx. 200 parts/h with Carousel Rotary Chamber shot blast systems or approx. 400 parts/h with Manipulator shot blast systems. Such systems have been used economically since around 1980.
An efficient system for shot peening of piston rods in single mode has not yet been realised, according to my information. By far, the largest volume of such workpieces is still processed in tumble-blast machines.
What I believe is this: It is easy to find a complicated and expensive solution, but it is difficult to find an intelligent solution for a simple machine execution that meets technical requirements and can be operated under economic conditions.
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Author: Albert Schlatter