Still no attatchment here, do you have a web link?
Nexen has a new revolutionary linear drive system that is similar to rack and pinion but eliminates it's short comings. This makes it a better choice for many applications that typically use traditional rack and pinion, ball screws, and linear motors. See attachment for details.
Last edited by allanconway2; 11-16-2004 at 05:02 PM. Reason: Add Attachment
Still no attatchment here, do you have a web link?
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)
There is also the 'Rolling-ring' drive that is interesting and has been around for a while, I think it is used mainly in lower load applications, I have come across them in CNC back gauges for shears & brakes etc , Amacoil is one manuf. http://www.amacoil.com/ due to the possibillity of slip under stall or high load, a linear scale is usually used to detect actual position.
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I picked "other" in the poll. I would have thought timing belts should have got a mention.
Thanks for posting the PDF file of the nexen drive system.
How comparable in price is it to other methods?
It look like It is made out of plastic. Is this correct?
Being outside the square !!!
Belt drives are fast and inexpensive, but not generally considered precise when it comes to linear positioning.
The roller pinion body and rack is made from high quality steel with excellent heat treat qualities. The pins are bearing grade steel. The black coating is Raydent. It is a high performance blackening process with excellent wear and corrosion properties.
Plastic racks may be considered in the future for low cost and force applications that need nonmagnetic, or high corrosive resistance.
Cost of the roller pinion system is comparable or less than ball screws and more expensive that traditional precision rack and pinion except for split and dual pinion systems that can be very expensive.
Last edited by allanconway2; 11-17-2004 at 12:06 PM.
Excuse my basic observation, but the literature claims the accurate positioning eliminates the need for "costly and complex split and dual pinion systems". I still imagine you would opt for rack and pinion on both sides of your gantry? I don't know if this is called dual pinion.
Dual pinion does not mean separate rack and pinion systems driving opposite sides of an application. Dual pinion systems have two pinions operating on a single rack and are preloaded against each other to eliminate backlash. This is done with either a single motor with two coupled gearboxes or two separate servo drives where the controller maintains the preload.
The attachment says it "eliminates the cumulative error problems experienced with ball screw systems". Could you please explain this further? It seems to me the cumulative error will consist of the accuracy with which the screw or rack is ground plus the thermal expansion.Originally Posted by allanconway2
In a ground and preloaded ballscrew with preloaded angular bearing block the precision is pretty much built into the unit at delivery. It seems that with your system some of the accuracy must be provided by the machine builder in that any guide system error perpendicular to the rack will affect the positioning error in the direction controlled by the rack/pinion. The effect of such errors are not described in the data sheet?
What is "Backlash Compliance". Ground and preloaded ballscrews have no backlash, but does have some compliance, usually expressed as a distance/force figure. What are the 3.2um? Are they backlash? Or are they compliance, if so at what force?
What about protection from the environment in which it is working. We're mostly concerned with processes generating swarf, dust, etc. here. What are your methods of keeping this debris from affecting the drive? When using ball screws there are usually 2 levels: An outer shield, often provided by the machine construction itself and the placement of the screw, a coiled spring or other shields. Then a secondary in the ends of the nut where many systems are used, some very effective, others not so.
Ball screw cumulative error is a widely recognized issue. There are numerous sources on the web that discuss this issue in depth. If you look at a ball screw accuracy graph you will see that the lines are not horizontal and angle away from the zero axis indicating a certain amount of lead error per unit of travel. Over short distances it is typically inconsequential, but over longer distances becomes a problem forcing the use of linear encoders to get accurate positional feedback. At lengths less than 9 inches ball screws are generally as accurate as the RPS system, but beyond this the RPS is much more accurate than a ball screw. As you have seen, the cumulative error graph for the RPS is horizontal meaning there is none regardless of the distance.
As for accuracy being built in to ball screws and guiding system design effecting the RPS systems accuracy. Precision and accuracy are not the same thing and you can have one without the other. Ball screws can be precisely manufactured but are not necessarily accurate. More on this can also be found by a web search. The guiding system requirements for the RPS are not significantly different than ball screws. Typically profile guide rails are used to guide the system and dictates the precision of component installation. Ball screws cannot tolerate side loading so the parallelism requirements of the system are high in either case. If the guiding system allows the RPS to loose preload due to poor machine design or installation then accuracy would be compromised.
On the issue of backlash you say that ball screws have none. This is not entirely true of ball screws or any mechanical system. Some clearance is always needed for parts to slide or rotate, although it can be very small. Even with bearings clearance is required for ball or needle lubrication or it would be metal on metal. Yes, mechanical nut preloading takes most of the play out of a ball screw but there is still clearance for lubrication that can be squeezed out when sufficiently loaded. If you look at so called zero backlash ball screw specs in a catalog you will typically still see a backlash value. The RPS uses a preload to achieve a similar effect. Equally important is system compliance or elasticity, ball screws are much more elastic due to deformation of the ball nut and the unsupported length of the screw. The 3.2um linear backlash stated for the RPS takes into account backlash and compliance. It was measured with high precision rotary and linear encoders while applying the maximum linear force while the pinion shaft was locked. In addition to this ball screws are much more susceptible to thermal expansion due to environmental variation and heat build up in the nut due to high speeds. The RPS rack is rigidly mounted and with it's 99% efficiency very little thermal expansion is generated regardless of how hard you run it
On the issue of environmental contaminates. The RPS system is susceptible to environmental contaminates as are ball screws and traditional rack and pinion. If the rack is not oriented with teeth up and ideally downward contamination is minimized. The lubrication requirements for the RPS are much lower than ball screws. The rack gets a light greasing at installation and then typically twice a year thereafter. Harsh environments may require more frequent servicing. There are many lubrication options from food grade to dry lube that can be chosen based on the application. Lubed for life ball screws aren't and are based on clean room conditions and light loads. The thing to keep in mind is that the needle bearings that support the pinion roller pins are permanently lubricated and sealed. As the rollers engage the rack it is mostly rolling friction. Each time a pinion roller engages and disengages a rack tooth it only turns a small fraction of a revolution. This gives the RPS its 99% efficiency rating compared to a ball screws 80%. In other words very little friction or wear takes place giving the RPS much longer life in most applications. We have had customers use the RPS for life issues alone and they are very satisfied with it. If contaminates get into a ball nut they quickly eat it up due to the high amount of work the balls do in the nut and opportunities for friction. As with any drive system if shielding can be installed to protect it from contaminates it is a wise thing to do and should extend the life of the system.
@Allanconway2: I appreciate your reply. Although I do not agree fully, your comments are good additional info to the data sheet.
Do you have any mounting suggestions with sketches/drawings showing preferred mounting and contaminate shielding?
Also it would be nice with some prices. I assume most in these forums are interested in one off prices for the hobby and small volume custom machine builder.
I would be interested in knowing what you disagree with. If you can show me that any of what I have said is in error I would appreciate it. I am not big on hyping things and only deal in facts as I know them.
For more information on installation I sugest you download the user/installation manual for the RPS from Nexen's website. Go to www.nexengroup.com. The RPS is presently featured on our home page and links are provided to various RPS information. I am not giving out price lists at this time but can give quotes on specific applications. Let me know wheather you prefer direct or through distribution and the quantities involved. Keep in mind that when comparing prices of the RPS to ballscrews that many aspects of the machine design may change and it is the final package cost that matters. From our experience the RPS is price competitive with precision ground ball screws and offers more value. If you are interested in seeing the RPS Nexen has a representative Belgium who may be able to meet with you.
The big linear guide manufacturers (INA etc) have guides with built in decoders, wich gives an absolutely no-backlash positioning. The only minor thing is - the price...
But at the last tech fair I visited there was a Chinese guide manufacturer that had the same solution but to a price that was reasonable. Still not cheap, but very interesting if precision is an issue.