[daedalus]

Hi,

Does anyone know where I can get very low static friction linear rail trucks? I don't need a lot of load capacity (100N or so), but the application is very sensitive to static friction. ideally I am looking for something in the ballpark of 50mN.

[RomanLini]

Those specs sound pretty weird, what is it for?

Something capable of driving a load of 100N would easily overcome such a small static friction from the bearing.

[daedalus]

Its for a custom human interface device for a computer. The static load of 100N is to provide some overload protection if its dropped/lent on, the truck will be moved by finger movements, so the human applies 0-10N force to the truck, and when applied force is low, the static friction can be perceived.

Normally these devices are build with rotary joints and steel cable-capstan drives, which can achieve the specified 0.05N static friction. I'm trying to figure out if I can do the same with linear motors and linear rail/trucks.

The problem I'm finding is that most of the catalogs I have checked don't seem to list static and rolling friction (I guess this is due to the seal choices changing the values).

[RomanLini]

You could try a seal-less dry linear bearing, I have an Igus Drylin 17mm (N series) linear rail and truck here, it is tiny and about the slipperiest thing I've ever seen in a solid bearing.

One tiny truck is rated for 25-50N.

You can see it here;
igus® Page not found

[wildwestpat]

If the requirement is for compound motion in two or more orthogonal axis then it is my experience that rotary motion is better. The linear slides have to support the twisting component of any compound movement and this adds to the stick-slip encountered in the motion experienced in a single axis or plane. IMHO try round slide pucks rather than rectangular trucks with the pucks arranged such that the input platform is supported by two rails per axis with just three pucks i.e. one puck in one rail with two pucks in the opposite support rail per axis. The aim being to ensure that the input platform does not jam / judder as the location in each axis is fixed by three points of contact with the restraining rails.

Other things to consider are the use of compressed air to float each axis and/ or magnetic levitation.

It is my experience that some reactive force is needed in a human machine interface to give a smooth controllable motion. This is counter intuitive but the addition of viscous damping gives better control. Damping greases or gels are available from various sources. Stick/slip - friction at the start of motion viscous damping are due to conditions at the contact area between the surfaces and each has an effect on the motion.

Good luck with the design - Pat