Linear Motion Control - 2014-02-12
Seals for dynamic applications have a dual function in drive elements. They are designed to prevent contaminants from entering into the interior of the components and, at the same time, ensure that no lubricant can flow out. In addition, in order to attain energy efficiency, very good sealing characteristics should be achieved while generating the least possible friction.
From the perspective of development (and physics), these conflicting objectives apply both for components in linear technology, i.e. for ball screws and roller guides, and for rolling bearings. Based on comprehensive engineering expertise and by using simulations, a very good solution to these conflicting objectives can be found as is demonstrated by the seal systems developed by NSK.
In the development of the A1 seal, the emphasis was on reducing the lubricant discharge from the nut in ball screws for high-load applications without significantly increasing friction. This objective was achieved and the result is offered in connection with the high-load ball screws in the HTF series.
A1: Seal concept for low lubricant discharge
In addition to the obvious environmental benefits, the A1 seal concept offers good economic advantages because in high-load applications, lubricants with EP additives are used, which can withstand high operating pressures. These additives drive up the costs for manufacturing the lubricant. For this reason, low discharge reduces the total cost of ownership for the operator.
Operating costs decrease even further because maintenance and lubrication intervals for the high-load spindles equipped with A1 seals can be reduced. Their very good grease retention properties counteract insufficient lubrication over the long term and in clean production environments (e.g. in injection moulding applications, where high-load ball screws are frequently used to replace hydraulic drives), low grease discharge is among user requirements.
V1: A seal for processing abrasive materials
Another seal concept was developed especially for a customer that constructs high-quality machine tools. More and more often, these machines are used to process graphite, which releases very fine, highly abrasive dusts, as well as ceramics and other sintered materials. This is associated with the risk that the released particles will damage or destroy conventional seals and, as a result, sliders and rails. For this reason, contaminants must be prevented from entering the contact zone between the guiding element and the slide.
For the roller guides in the RA series, NSK developed the V1 seal, which is manufactured from an extremely abrasion-resistant material and which has a sealing lip that moulds itself into the roller guide profile with no gap. In combination with a cover strip, this design meets the requirements for precise ‘task sharing’ between wiping and sealing. The result: in comparison with a standard seal, the penetration of particles was reduced by a factor of ten. And the grease retention properties were significantly increased as well.
The V1 seal concept is particularly effective. It is now also offered for use with ball screws in combination with the optional K1 lubrication unit. In this case, the lubricant is incorporated in an impregnated plastic material that dispenses low amounts of oil upon contact with the rolling surface. This provides additional support so that the linear drive system runs very smoothly with low friction. A series of investigations by NSK prove that the K1 lubrication unit also supports sealing behaviour.
Design unit comprising seal, lubrication and covering in one
The third design element for these drive elements, which achieve high service life and provide security against downtime even under extreme conditions and when processing abrasive materials, is the covering strip. It prevents contamination from being deposited in the mounting bores and, at the same time, ensures that the surface on which the seal moves is perfectly even. This demonstrates that it is best when seals for dynamic applications are considered in combination with lubrication and covering and the design is adapted with respect to all of these elements.
NSK have also developed another seal concept for ball screws especially for machine tool engineering. Here designers were able to use the basic design of the V1 and A1 sealing concepts.
With the X1 seal, an even better seal was achieved in comparison with the V1 seal in that the two basic functions – preventing dirt penetration and lubrication discharge – were separated into two distinct seal elements. An external wiper covering removes particles from the spindle to prevent contamination from reaching the rolling contact zone. A second seal has the exclusive task of retaining the grease. With this property profile, the X1 seal concept represents a real milestone in the advanced development of high-performance seals for ball screws.
The objectives listed in the specifications were exceeded in practice. A wiping function ten times better than before had been planned. The value measured was about 32 times better. Grease discharge was also significantly improved: less than one-third of the otherwise normal amount of lubricant was discharged. Clearly, the seal concept reduces lubricant consumption. Because there is less friction, it also supports smooth running properties and, consequently, the efficiency of precision drives.
Effective seal concepts in automotive development
There are also effective, energy-efficient seal concepts for rotary bearings. In this case, NSK can rely in part on the synergetic effects from the development of automotive rolling bearings because in the automotive industry, intensive and successful work is being done to reduce friction for all drive components.