in Vol. 9 - July Issue - Year 2008
Shot Peening of Coil Springs: Modern manufacturing technology for innovative products
Picture 1: Coil springs
Picture 2: Peening principle using blast wheels
Picture 3: RDS Mini for small springs
Picture 4: Example of loading system
Picture 5: Valve springs
Shot peening is an established technology which is optimally designed to enhance the fatigue strength of components subject to high alternating stress. Shot peening makes for lighter components which, optimized with respect to weight and cost / benefit, can be loaded up to the limits of their properties. Shot peening is cost effective and reliable. It is often the sole treatment method or substitutes complicated, cost-intensive procedures and can be applied practically independent of shape and size of the work-pieces – thus permitting an appropriately wide field of application. DISA Shot Peening Systems using blast wheels offer all prerequisites for the reliable process-safe treatment of work-pieces of all kinds, geometries, sizes and qualities. Systems made by DISA are flexible, in particular if process or production requirements change and thus offer long-term investment security.
Shot peening using blast wheels is in particular suitable and appropriate when large series of mass products - like coil springs - are produced and processed. The blast wheel technology offers economical and technical advantages under such conditions. Test experience has shown that the fatigue strength of springs can be increased by 20 – 100 %, however it is mandatory that appropriate equipment (machine, blast wheels) with carefully tailored blast patterns and appropriate shot - usually cut wire, pre-conditioned G2 or G3, hardness 640 HV – are used.
Shot peening of single coil springs in continuous operation
With RDS Shot Peening Systems, single coil springs (suspension springs, axial and valve springs) of approximately 20 – 250 mm in diameter and lengths of approximately 35 – 600 mm are shot peened in longitudinal, rotary throughput principle. Blasting in throughput operation is simple and safe. It allows process-safe shot peening where part tracking is possible and is ideally suitable for automatic production lines with continuous flow of parts without intermediate buffering. Adjustable throughput speeds (and dwell times) ensure process safe defined peening in uniform quality. Automatic systems with high performance and manufacturing consistency reduce production cost. Once integrated into automatic procedures the installation only requires periodical inspection, and operating expenses can be reduced.
In-line with the different work-piece dimensions, DISA is applying two basic machine types of the RDS Line which includes various type sizes and is using the same operating and blasting principle. Flow-rates depend on relevant work-piece sizes, shot peening requirements (Almen values, coverage) and loading / unloading systems. Approximate production rates are:
• up to 1’200 suspension springs per hour
• up to 5’000 valve springs per hour
RDS Shot Peening Systems
A specific feature of the RDS Shot Peening System is its ability to handle coil springs in a wide range of dimensions including small parts and to treat them individually under uniform and defined conditions in a continuous process. With suitably adapted loading and unloading devices, the shot peening process can be integrated into continuous production lines.
Coil springs are fed through the blasting zone on horizontally rotating carrier rollers. The axial movement is implemented via carriers which are attached to an endless transport chain. The transport speed is infinitely variable within certain limitations. The parameters of the shot peening process - shot quantity, blast wheel speed/throwing velocity, the speed at which the work pieces rotate and the dwell time, can all be regulated to suit the requirements of a specific type of work piece and lead to the required coverage. It is this definition of all treatment parameters that ensures process-safety at all times. Process parameters can be adjusted according to requirements and recorded in the control system and retrieved when necessary. The shot peening programs for the different applications are assigned to the respective part families, ensuring that the given parameters are used in operation. The individual programs can be activated via the appropriate settings at the operator’s panel.
Process-safe shot peening of valve and small pressure springs
Valve springs or small pressure springs, are typical work pieces whose endurance and service life is significantly increased by shot peening. Small metal work pieces (valve and pressure springs) can in fact be shot peened in bulk on trough-shaped barrel belts, however, this method does not provide process safety as nowadays often required.
In RDS-Mini Shot Peening Systems, continuously rotating, single work pieces travel on horizontal rollers through the machine. The machines are designed to strengthen valve springs and suspension springs by shot peening at a throughput rate of more than 5’000 springs an hour, depending on relevant requirements. The springs are fed to the machine individually on a straight-line conveyor or other loading systems and then move through the blasting zone on continuously rotating horizontal rollers. Axial movements are effected by cams attached to chains.
Inside the blasting zone the springs are properly guided by adjustable baffle plates which also serve to focus the shot stream on the work pieces for optimal exposure within the “hot spot” of the blast pattern.
The working principle of this machine series is the same but due to size and weight of the work-pieces, no baffle plates are required. In contrary to the RDS Mini - this machine type can be executed as a single or double work-piece strand machine with 2 or 4 parallel rollers and with 2 or 4 roof mounted blast wheels. Production output can thus practically be doubled and overall efficiency is of course increased significantly. The concept of peripheral equipment corresponds to the various requirements: Loading and unloading can be either manual, via feed modules, manipulators or with industrial robots.
Dust removal and shot reconditioning
A dust separation system (we principally recommend the use of separate filters for each individual shot peening system) allowing the function of the pneumatic separation is incorporated with every shot peening machine. It fulfils the following functions:
• Separation and discharge of scale and dust from the abrasive
• Elimination of undersized abrasive which is no longer suitable to maintain a proper shot peening function (i.e. no need of a grain classifier)
As the life time of the wear parts depend primarily on the degree of purity of the abrasive, the following is applicable:
• The better the separator, the higher it’s profitability
• The cleaner the abrasive, the cleaner and more dust free the work-pieces
Besides the wear and tear issue, the abrasive consistency in the distribution of the grain size is – in particular for shot peening – a key criterion in terms of quality oriented manufacturing. Loss in quality or insufficient shot peening results (such as surface hardening) can thus be excluded. New shot is automatically fed into the cycle via an electronically controlled replenisher.
• Shot peening using blast wheels and based on the throughput principle is simple and process-safe, suitable for automatic production lines with a continuous work piece flow without intermediate storage.
• Individually adjustable parameters ensure work piece-specific shot peening of the desired quality.
• Automatic systems of high performance and manufacturing consistency reduce production costs.
• A solid machine structure and high-quality machine components ensure long service life and low maintenance costs.