Surface treatments for longer wearing machine elements
08 August 2013
The footprint of any machine is very important and, for that reason elements are made as small as possible, compatible with the task that they are required to perform. Roger Haw explains the considerations that need to be taken into account by designers when choosing surface hardening for such components.
Most machinery, for whichever purpose it is made, is complex and usually consists of elements that either move at high speed, are subject to wear, or subject to cyclical loading.
In this article, we will be looking at equipment such as bottling, wrapping, pharmaceutical and textile machinery, and automatic assembly machines, where the heat treatment of parts is totally different to that applied to components found in equipment for the mining, construction, automotive, marine and similar industries.
Designers of machine elements usually look to surface hardening to increase resistance to wear on certain areas of the component, and the obvious solution is either to apply a wear resistant coating or to harden the component in the required area.
In certain applications, however, a coating is not acceptable; for instance, it may introduce wear debris into food manufacture. In most cases, therefore, heat treatment of the component is the only solution and the preferable methods of treatment are those in which specific areas are selectively hardened. Consideration must also be given to any possible distortion that may be caused by such treatment.
The range of metallic materials available for the production of machine components includes low alloy steels, medium carbon steels, SG irons, stainless steels, and steel strip, which are suitable for parts as diverse as cams, slideways, levers and transmission components.
Each type of item requires a treatment that will improve the part, but without making it either very expensive or impossible to fit into place after treatment. For such components designers often choose either flame or induction hardening. Flame hardening is more suitable for very small quantity or one-off requirements, whereas induction hardening is more suited to regular batches, both large and small.
The advantage of selectively surface hardening is that it will keep distortion down to a very minimum and often produce no distortion at all. For example, a simple trigger or lever may only need hardening applied to the area that bears against the mating part, which often involves heating a volume of material less than 2 percent of the total volume.
A slideway may require only the wear surfaces to be hardened. In this case careful consideration must be given to the method of manufacture as most slideways are designed to be much longer than their width and depth. In such cases stress relieving should be considered at an intermediate machining stage, prior to finishing to the hardening sizes, and hardening should be restricted to as little depth as possible.
In some cases through-hardening is required on thin sections. The grippers and reeds found in textile machinery are an obvious example, being made of material usually only 1mm thick.
Flame and induction hardening techniques applied to small machinery elements is extremely precise and cost effective, and bespoke treatments can easily be devised to suit the product and the application.
However, the key to success in all of this lies in the design and manufacture of heat treatment tooling, which, like the metallurgy, requires an in-depth knowledge when it comes to the selection and treatment of specific materials.
Experience plays an important role in the design and production of this tooling. A problem that has been solved on a previous project will often provide the basis of a solution for the treatment of a new part.
Roger Haw is managing director of Flame Hardeners
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