Heat treatment is commonly used to prolong the life of a ball screw. During the heat treatment process, the outside layer of both the ball screw and ballnut are hardened, ensuring a predictable life, especially in high load applications. Without this layer of hardening, the ball bearings used in a ball screw would break down the surface of the assembly, resulting in failures and downtime.
How are screws heat treated?
In the PST screw production process, centerless grinding, thread rolling, and heat treatment are all in-line, to ensure that all sizes are met after thread rolling. Heat treatment of a screw can be performed at various different times throughout the process, depending on requirements of the assembly.
During the heat treatment process, a ball screw passes through an induction heat treater using different sized coils, various coolants, and drive speeds to ensure that the proper depth and hardness is met. The PST Group uses a micro-hardness tester during the setup process to verify that all of these specifications are met.
With thread rolled ball screws, heat treatment is typically performed after the thread rolling process, as the hardness level of the ball screw does not allow for thread rolling after induction heat treatment. However, the timing of heat treatment can vary when using a thread ground ball screw. This is because thread grinding allows the thread to be cut even after heat treating, it is preferred that heat treatment is performed prior to grinding, to ensure that the sizes and lead accuracy will not change due to the heat treatment.
What are the benefits of heat treating a screw?
PST provides heat treating for both ball screw assemblies and acme screw assemblies. Heat treatment promotes a longer life expectancy in acme screws just as in ball screws, however it also increases the toughness of the assembly. The hardness value is generally much lower in an acme screw than a ball screw assembly to promote the toughness required within the assembly.
With acme screws, typically the shaft of the screw is hardened, while the nut is made of a softer material, such as bronze or plastic. All ball screw assemblies are heat treated, as they require a high hardness to meet dynamic load capacities. Acme screws do not require the same hardness value, though heat treatment will provide additional hardness if needed.
This heat treatment process will vary depending on the ball screw material. For carbon steel or alloy steel ball screws, induction hardening and case hardening is the recommended heat treatment process to achieve the high hardness necessary for the application. For stainless materials, PST uses outside sources to achieve the required hardness. However, the hardness value of stainless materials will not be that of carbon steel, resulting in a lower dynamic capacity and shorter life expectancy.
PST Heat Treatment Capabilities
The PST Group’s in-house heat treatment capabilities are focused on induction heat treatment of ball screws. PST utilizes a horizontal induction heat treatment process that can accommodate lengths of up to 40’ and diameters up to 4”.
If a different heat treatment method is required, PST works with a variety of heat treatment specialists throughout the greater Milwaukee and Chicago areas to provide customers with exactly the specifications they need. If a special heat treatment process is required, PST is not hesitant to ship parts across the US to meet the customer’s requirements.
Interested in learning more? Contact us below and one of our heat treatment specialists will be in touch soon.
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