Wipers wipe – right? Well, not always. As a matter of fact, many ball screws that come with “wipers” at both ends of the nut have plastic rings that don’t touch the screw surface. So they don’t wipe. They just reduce the gap between nut and screw shaft to a few tenths of a mm. And that’s ok, because most ball screws that are used in machine tools will be connected to an automatic oil (or liquid grease) supply, and the used lubricant must get out of the nut, somehow. Offering it a constant small gap means there will be controlled flushing, and that’s good – for most applications.
When it comes to real wiping because there is too much dirt present, or because there is limited or no further lubricant available once the ball screw has been installed, look for “contact wipers”. And things get complicated here.
Once you have contact between the wiper and the screw shaft, there will be additional friction as well. And if you require a certain degree of sealing, you will need a certain preload of the wiper which, along with the coefficient of friction, determines the amount of friction. It does help to concentrate the force that presses the wiper against the screw shaft surface in a small area, so we should really talk about contact pressure in the technical sense.
Of course you should consider that a ball screw seal always has to be quite sturdy compared to the lip seal of a ball bearing. The reason is simple: there is an axial component of the movement, which causes a sideways load on the lip of the seal. So it can’t be too flimsy. Ball screw (or ball nut) sealing will always be a trade-off between friction (which is bad because it means additional heating) and contact pressure (which is good because it keeps the lips of the seal efficiently “plowing” whatever matter you try to keep from getting inside the ball nut).
Another issue with contact seals is the risk that the friction will vary with time. Shrinking of the wiper material from temperature variations, or from aging of the plastic, can be catastrophic. Contact seals normally need to include some spring to keep forces more or less constant.
A special seal that has a second funtion is made from felt or custom plastic material, and saturated with oil. Such seals offer limited additional lubricant reservoirs and can be quite useful to extend the service interval of screws that are not connected to an oil or grease pump. But of course their use must be reviewed carefully. The amount of lubricant that comes from the seal is quite limited, and lubricant starvation with all of its consequences may be the result. As there is always a connection between the quantity of lubricant flowing through the nut and the amount of foreign matter that can be dealt with, small oil (or grease) flow means that perfect sealing against contamination is a must. So things get tricky when only small amounts of lubricant are available.
The felt has several advantages here: it stores more lubricant than the oil saturated plastic; it is capable of absorbing particles and thus removes them from the system; it can be re-filled; and it lets you choose the lubricant best suited for the application. The impregnated plastic on the other hand is limited to whatever oil they used when making it, plus it needs a lot of pressure (and thus actually some heat) to release the oil. And it is less effective against fine particles.