The name of the game for machine builders nowadays is maintenance-free precision. End users are demanding it for their products, to avoid warranty hassles or plant breakdowns. Servo-controlled linear drive systems have risen to the challenge, with increasingly durable ballscrew, belt, and linear motor technologies. That said, each of these systems has strengths and weaknesses, so none is ideal for all applications.
Critical and maximum speeds, cumulative error, vibration, thermal expansion, and supplemental cooling requirements are the ballscrew's drawbacks; belt drives exhibit lower accuracy and load capacity, backlash, short strokes, belt stretch, and low rigidity. Costly linear motors also have lower load capacities, as well as strong magnetic fields that can interfere with nearby systems and controls. In extreme cases, these motors can also require liquid cooling. Traditional rack-and-pinion systems are limited by low accuracy and speed, tooth fatigue, and noise. Costly dual and split pinion systems are required to improve positional accuracy and reduce backlash.
New roller pinion systems (RPSs) are one linear drive option with fewer drawbacks.
How it works
An RPS is like a traditional rack and pinion set, but instead of a spur gear, a pinion of bearing-supported rollers engages a uniquely matched rack instead. The rollers ride smoothly along the face of each rack tooth. So, there's none of the sliding friction of traditional rack and pinion sets. Smooth rolling friction makes for 99% efficient rotary-to-linear motion conversion.
The RPS tooth design is also different from traditional pinion designs in that it behaves like a cam and follower. Specifically, cycloidal curves are created when points on the pinion roll on the rack without slipping. Normally, this concept exhibits backlash, but a technical modification of rack tooth geometry allows two rollers to remain loaded in opposition at all times, eliminating backlash as the rollers engage the rack. So, the rollers meet the rack smoothly, without the tooth slap, sliding friction, fatigue, low precision, and noise associated with traditional rack and pinion.
Keeping quiet
The system is nearly silent at low speeds, and produces less than 75 dB at full speed. Typically, the guiding system, servomotor, and reducer are louder. Less noise and vibration increases accuracy and improves the working environment for personnel who may have to work near the machinery.
Setup and life
Installing an RPS is similar to the installation of precision profile guide rails: Alignment is key. The rack should be placed on a step in the machine bed for full bottom support, a back plane for bolting it in place, and to ensure straightness — particularly when joining rack sections. Once the pinion is mounted, its axis positioned parallel to the rack tooth faces, and fully engaged in the rack, a small preload takes up clearance in the pinion roller bearings. A very flat rack mounting surface helps here, but more important is the parallelism between the guiding system and the RPS, so that pinion preload is neither lost nor excessive.
RPSs require little maintenance. The pinion consists of 10 or 12 needle-bearing-supported rollers that are sealed and lubricated for life. The rack is lubricated with light grease at installation, and then every six months or 2 million pinion revolutions. In special applications, for a small sacrifice in system life, the RPS system can also be run without lubrication. One caveat here: Speed must not exceed 30 m/min. In contrast, other mechanical linear drive systems require more frequent lubrication; only a linear motor requires less.
Going without lubrication can also be beneficial in dirty applications such as cutting, milling, and routing. Contaminants are less likely to stick to the rack, reducing the creation of an abrasive paste that accelerates wear in mechanical systems.
A special coating helps the RPS excel in many applications. RPS rack and pinion bodies are made from carbon steel, and the rack teeth heat treated for wear resistance. Now, in many applications, liquids and corrosive materials would pose a problem for an unprotected steel product. Stainless steel is not viable because it has less strength.
So, often a protective surface treatment called Raydent is applied. It permeates the metal surface and molecularly bonds with the steel to form a durable ceramic-chrome layer while causing minimal surface buildup. Highly resistant to acids, alkalis, and solvents, it doesn't flake or rust if scratched. Even if a thin piece of metal is treated with Raydent and then sharply bent in half, the coating is not compromised.
Because lubrication requirements are modest or nonexistent, RPSs exhibit very low particle emissions, making them suitable for clean rooms, food processing, coating operations, and pharmaceutical production. And RPS systems provide 60,000,000 pinion revolutions of life at its rated performance, for 9.6 to 28.8 million meters when properly installed. (The RPS system continues to operate beyond this point with diminishing accuracy.)
Now, many linear drive technologies are sized for life considerations, with product size increasing with life requirements. The RPS system selection process does not take life into consideration; it is primarily based on load. Let's explore this process further.
Selection: Directions and example
First off, to pick a properly sized RPS, collect your application data carefully. These specifications are needed to determine load mass, acceleration, force due to acceleration, force from gravity and friction, and total force of the load.
We'll now go over some sample calculations. See Example calculations figure, right.
A guide with sliding friction typically has a friction coefficient value between 0.1 and 0.2. Rolling element guides, on the other hand, exhibit a friction coefficient of 0.005 to 0.02. Shock factor K indicates the smoothness of operation: This value is 1.0 to 1.2 for perfectly smooth operation, and 1.2 to 1.5 for normal operation. If the system will endure impact during operation, assume a K value between 1.5 and 2.5.
Calculated weight to be driven should include the servomotor, reducer, guide rail bearings, and platform when applicable. Also, other forces may include springs, counter balances, fluid dampening systems, wind resistance, and so on.
If the acceleration and deceleration times are different, or there are other changes in velocity over the run, calculate the acceleration forces for each interval and use the highest one for RPS selection purposes. Compare the total force calculation to the maximum dynamic force ratings in specifications tables to select the ideal RPS size. Then verify maximum speed.
More on accuracy
Even the small individual pin and rack meshing errors and general periodic wave error in the pinion body that repeats with each revolution does not accumulate.
RPS racks are modular, so standard factory lengths can be joined to create longer runs. (Full lengths are around one meter in length and half-lengths around a half-meter in length depending on tooth pitch.) Likewise, cutting sections down can make shorter rack segments. But assembly is more than simply butting racks end to end; an alignment tool is used to engage the rack tooth profile, to set rack spacing, in a way that maintains positional accuracy over multiple rack segments. The alignment tool uses two teeth on each rack to average out transferred error. This small error is added and subtracted each time, to statistically trend to zero, which means that an infinitely long run has, in theory, zero cumulative error. This allows runs of virtually any length without loss of system accuracy. In many cases, linear encoders can be eliminated, and a simpler, more cost-effective servo encoder can be used instead.
RPSs maintain accurate positioning at speeds as high as 11 m/sec — normally only reached by linear motors. In contrast, ballscrew speed capabilities are much lower, and diminish with length. Even at these speeds, the extremely low-friction design does not create heat or wear on components. The RPS meshing action provides backlash of less than 3.2 µm and positional accuracy up to ± 20 µm.
RPS technology eliminates the cumulative error and thermal expansion error problems of ballscrew systems. It also eradicates backlash in both directions by maintaining opposing contact with two or more teeth at all times. This does away with the costly and complex split and dual pinion systems required by most traditional rack and pinion systems to achieve zero backlash.
For complete specification instructions, more on life ratings, and current information, visit nexengroup.com or call (800) 843-7445.