Acme screws, which convert rotary into linear motion, are quickly becoming the preferred choice for design engineers among numerous motion control applications for a multitude of reasons. Acme screws are affordable. They are estimated to offer a cost savings of up to 85 percent compared to alternative motion control components. They are also precise, flexible, and easy to maintain for long-lasting success.
Many features make acme screws desirable, but it's worth noting that many important factors need to be considered when making your screw selection. This post will take a look at some of these:
Backlash: Backlash is the relative axial clearance between a screw and a nut. This clearance is necessary for an Acme nut to move freely along a screw. Backlash always increases with use as parts wear and can be detrimental in precision applications where positioning is critical. Hence, acme lead screw designs need to compensate for backlash so that inaccurate positioning doesn't occur.
So how do you reduce or remove the backlash between the screw and the nut? You have two nuts that are pushed apart, so they are each biased to the opposite flanks of the screw thread. There are several methods that each have trade-offs between load handling and drag torque. Helix has several options and can help you decide which one is best for your application.
Load Evaluation: When selecting an acme screw, another essential consideration is the load that it will be working with. Excessive loading can lead to a shorter lifespan or complete failure. Five types of loads impact screw assembly performance: tension, compression, thrust, overturning, and side. Get familiar with each of these load types in our catalog and determine which ones your screw is likely to experience. It's also important to familiarize yourself with the load classes, including static, dynamic, and PV.
Lubrication: Acme screws are generally far less cumbersome to maintain than competing technologies. However, a big part of ensuring their long-lasting, maintenance-free life is choosing the right lubrication for your application. Acme screws do experience friction when rotary motion is converted to linear motion. However, the right lubrication not only reduces this friction, but limits wear, reduces maintenance, and maximizes the life of the Acme nut.
Aside from friction reduction, lubrication also transfers heat, carries away debris and contaminants, and guards against corrosion. There are two main types of lubricants used with Acme lead screws: liquid and non-liquid. The former generally consists of petroleum-based oils that don't chemically react and can operate at high temperatures. Conversely, non-liquid (or dry) lubricant uses powder where the liquid lubricant is ineffective or not practical. Helix Linear Technologies uses proprietary, specially engineered resins that have lubricants integrated into them. The polymer nut transfers some of the lubricants to the screw. This allows for a low coefficient of friction for the screw nut combination. The lower the coefficient of friction, the lower the drive torque is required and the longer the life.
While acme screws are a massive player in linear motion applications, there's a lot to consider when selecting the right one for any particular application. Failure to carefully analyze the necessary accuracy, all the loads present, and the type of lubrication could lead to an application that's more likely to fail.