Using Airpot’s Piston-Cylinders as Sliding Linear Bearings?

We use our graphite pistons and glass cylinders as the core technology to make our air dashpots and low friction air cylinders. Some customers go in a different direction and use the piston-cylinders to make sliding linear bearings. Let’s talk about some of the issues involved with this.

A rod in a precision V-block is a classic way to create a sliding linear guide. However, there are many problems with this including high moving mass, lubrication problems, and how to maintain the nesting force to hold the rod in the V-block.

Airpot’s graphite piston has a density of about 25% of steel, and it’s self lubricating, so grease or oil is not needed. That helps keep the assembly cleaner plus there are no worries regarding re-lubrication. No nesting force is required because the piston is surrounded by the cylinder. However, since the piston must move freely in the cylinder, there must be non-zero clearance between the piston outside diameter and cylinder inside diameter.

You might be thinking that a single piston fit with clearance to the cylinder will tilt too much in the cylinder to be useful as a bearing. Let’s take a look at a real world example below but with the piston to cylinder clearance greatly exaggerated so we can better see the tilt angle.

The tilt angle is calculated as sin^-1(h/x).

Running the numbers for a typical 24 mm bore Airpot cylinder and a 12 mm long piston gives about 0.05° of tilt. This is the simplest design and may work for some applications. However, many are much more demanding. The tilt can be reduced by fitting the piston closer to the cylinder (reducing h in the equation) but you quickly run into manufacturing limits as we are fitting the piston as close as we can while still keeping the friction low.

A better way is to use a longer piston (increasing x in the equation). However, Airpot pistons are molded to specific lengths and can’t easily be made longer.

The first solution is to bond two pistons together as shown below. Airpot has the technology to bond the pistons so they are co-axial. It’s good practice to relieve the center of the piston assembly so it bears nearly on its full length. If you double the length of the piston like this you roughly cut the tilt angle in half.

A second solution would be to add a rigid spacer between the two pistons. In this example, this technique gives an effective piston length (x) of 80 mm. This could greatly reduce the tilt angle in the example to less than 0.01°.

Of course there are many other considerations when designing a linear bearing such as attaching the payload to the piston assembly but I’ll leave that for another blog post.