Productivity Forum: Rotary to linear converters

May 1, 2002
Rotary motors are the workhorses of industry, but their job would only be half done were it not for ball screws, lead screws, belt drives, and rack-and-pinion systems. Without these devices, industry would grind to a halt. Hear what our distinguished panel has to say about these essential components, and what you can do to maximize productivity through them in your next application

What elements most often limit machine productivity?

Paul/Specialty Motions: In ball screws, the rolling elements are almost exclusively the component that fails. In lead screws, it’s the wearing of the nut, due to friction, that typically causes failure. Other factors include lack of maintenance (cleaning, lubricating, etc.), harsh environments (debris, caustic chemicals, extreme temperatures, etc.), improper installation (misalignment), impact loading, and incorrect sizing.

John/Amacoil: The bearings themselves limit machine speed and load. Friction is also a problem. Speeds are usually limited by the heat build up caused by friction; drive accuracy is also affected, worsening over time due to friction-induced wear.

William/Parker-Hannifin/Daedal Div.: In precision tables, linear speed is limited by the lead and maximum rotation speed of the drive screw. Bearing support may also be a factor in terms of the radius of gyration. In general, maximum rotation speed decreases with increasing travel lengths. Belt drives and rack-and-pinion systems, on the other hand, are limited in speed by size (mass) and length. With belts, sag and stretching are also a concern especially in terms of positioning accuracy and repeatability. In general, the longer the travel, the worse the sag. Stretch comes with continuous use, although it can be offset by preventative maintenance and a few well-placed tensioners.

John/Exlar: Maximum linear speed is a common constraint. In screw mechanisms, maximum speed is dictated by screw shaft diameter, length, and the dn value of the nut assembly.

George/Steinmeyer: There are several limiting factors. First, there’s the dn rating of a ball nut, given by its diameter times its maximum rotational speed. Second, every rotating screw has a critical speed, determined by diameter, bearing support arrangement, and length. Some design options, including switching to a rotating nut configuration, can alleviate this limitation. Third, failure usually occurs from material fatigue; that is, the recirculating balls eventually wear out due to the classic Hertzian pressure of an applied load. Other factors include abrasion due to contamination and adhesion due to microwelding from loss of lubricant. Failure here is defined as loss of preload, resulting in performance degradation.

James/THK America: Friction is still the main enemy of industry. In an assembly line, objects are in motion. The more friction present in the system, the more energy and, accordingly, expense it takes to move a particular item from point A to point B. Unfortunately, if there is physical contact between two rolling or sliding elements, sooner or later they will wear out. Ball bearing raceways will experience burnnelling (surface cracking), belt drives will lose their pretension and experience tooth wear, and so on. These effects can be compounded by applied loads, moments, accelerations, and other forces that resist a change in motion.

Tom/Kerk Motion Products: Material failure is the most common mechanism — sometimes expected, sometimes premature. Lubrication, if required, is another weak link. And so is heat. However, the most common limit to increased machine productivity is cost, not failure. Return on investment and budget constraints often preclude the initial investment required to achieve the level of automation necessary for increased output and lower unit cost.

Wayne/SKF Motion Technologies: Misalignment — if it results in a bending stress — is almost certain to cause premature failure in ball and roller screws. Bending introduces radial loads the components are not intended to handle. (Ball and roller screws are designed for axial loads.) Lack of maintenance is also a concern, adversely affecting both performance and life.

Robert/Danaher: Maximum screw speed — the so-called critical speed — is one obstacle to productivity, setting a lower limit on cycle time. Another is duty cycle, how often a process may be repeated without exceeding heat or precision constraints within the system.

Michael/Joyce/Dayton: Speed, duty cycle, and load carrying ability are all limiting factors in machine productivity. Where screw jacks are used, the most common problem is the failure of the worm gear drive.

Andy/Rockford Ball Screw: The element that tends to wear out or fail first is lubrication. Lack of or failure to lubricate can ruin many types of rotary to linear devices and their supporting bearings. Special environments such as clean room, vacuum, high and low temperatures, and radiation make the lubrication challenge even tougher.

What is “productivity” and how do rotary to linear converters contribute to it?

Paul/Specialty Motions: Productivity is the ability to manufacture product efficiently. Rotary to linear devices contribute to productivity by automating processes to increase output and provide accuracies that cannot be reached by manual methods.

William/Parker-Hannifin/Daedal Div.: Productivity is a measure of the amount of work that adds value to the end product. It can be realized through two improvements — precision and speed. High precision components and machines reduce the amount of time lost to human error. If accurate and repeatable, the same devices also eliminate waste, improve quality, and make it possible to more efficiently re-allocate labor. Speed, on the other hand, reduces manufacturing time by increasing part transfer rates, packaging and palletizing, and by eliminating unnecessary personnel.

John/Amacoil: Productivity is increasing throughput while decreasing downtime. In winding applications, reducing set-up time is one way to make machines more productive. Here, rotary to linear converters (variable pitch drives with adjustable stroke lengths) can play a major role, eliminating the need to re-synchronize take-up drive motors with each new pay off feed rate. In spraying operations, on the other hand, productivity is more a matter of eliminating rejections and rework. The key here is maintaining consistent paint film thickness, which calls for an actuator that runs at uniform speed and reverses instantaneously at the end of the stroke.

John/Exlar: Productivity is a measure of product produced or processes completed per unit of time. This includes operating time, set up time, and down time.

Tom/Kerk Motion Products: Productivity is often expressed in units produced in a specific time period or for a specific cost. Focusing on cost, rotary prime movers are very economical for variable speed and/or distance, and when combined with a rotary to linear converter, such as a lead screw or belt drive, almost any desired linear positioning system can be created inexpensively. Wayne/SKF Motion Technologies: Rotary to linear devices are most productive when they operate at the highest possible efficiency with the least lost motion. Michael/Joyce/Dayton: For screw jacks, productivity lies in their ability to maintain accurate, repeatable action without loss of position.

Robert/Danaher: Ball screws improve productivity by optimizing efficiency (some are 95% efficient) as well as precision.

George/Steinmeyer: Productivity is typically output per given unit of time. The recent introduction of ball screws with dn ratings of 160,000 (33% higher than before) now means that a 40-mm diameter screw can operate at 4,000 rpm, significantly increasing productivity.

What can component MANUFACTURERS do to offset limitations and increase productivity?

Robert/Danaher: For screws, the problems associated with high speed and high duty cycles can be alleviated through the use of dry film lubricants; in cases where lubrication is not possible, engineered polymers — which increase the PV limit of the nut/screw system, while reducing the wear rate — are helpful as are new materials such as ceramic bearings in ball nuts. These, as well as more mundane approaches such as higher lead values, can improve high-speed performance, extend useful life, and reduce or even eliminate the need for lubricants.

Paul/Specialty Motions: Manufacturers have come up with many solutions to minimize the effects of “real life” applications where environments are not controlled, maintenance is not possible, and errors occur in assembly. Seals, wipers, and bellows have been designed to help eliminate contamination by debris. Materials such as stainless steel and coatings such as Raydent and Teflon are used to reduce friction and protect components in harsh environments such as wash down applications. Some ball screws are available with self-lubricating options for applications where scheduled maintenance is not practical. You can also find ball screw and lead screw nuts that have been designed to flex to compensate for misalignment.

Andy/Rockford Ball Screw: Manufacturers are currently employing different types of plating and lubricants to increase lubrication life and thus system life in general. The types of plating currently used to increase life would be various types of phosphate coating, fluoride chrome plating, and certain types of nickel plating. These types of plating, when used with the proper grease, can significantly increase the life of many types of rotary to linear devices. There are also new seals for ball screws that can retain lubricant longer than they have in the past.

John/Exlar: One solution is to use planetary roller screws because they offer higher dn values than other screw mechanisms. This means higher rotational speeds and thus higher linear speeds.

James/THK America: Traditional linear and rotary bearings suffer from ball-to-ball contact, resulting in higher levels of noise, contact stress, particle generation, and greater drive torque ripple. This can now be fixed, however, with special retainers that “lock” each ball element. By reducing friction between rolling elements, the retainers extend component life, minimizing heat as well as the number of airborne particles generated during operation. Other benefits include higher speed and smoother, quieter motion. The technology is also virtually maintenance free because it employs a grease “pocket” to separate each ball.

George/Steinmeyer: To extend ball screw life, two issues need to be addressed: prevent contamination from entering the ball nut, and ensure proper lubrication during operation. Combination wipers are one solution, having plastic fingers that sweep away external dirt and particles, along with felt reservoirs that continuously apply a thin lubricant film.

John/Norco: Customizing drive mechanisms using different materials to meet specific application needs is one approach. Making it easy to replace parts prone to wear is another.

Michael/Joyce/Dayton: Helping designers understand product limitations is the place to start; analysis software that lets users evaluate different solutions is the key.

Bernard/Haydon Switch & Instrument: Manufacturers can focus on proper material and process selection. For example, molded thermoplastic drive nuts outlast metallic bronze nuts by 100 to 1.

Tom/Kerk Motion Products: Addressing the maintenance problem has great promise. Providing extended life without the need for maintenance can easily add 5 to 10% uptime. Additionally, overall product life can be increased two to five times.

Why hasn’t this been done before?

Robert/Danaher: To a great extent, advances in materials pace advances in screw and actuator performance. Then, too, application of advanced materials requires extended qualification testing in specific applications, which takes time. Furthermore, mechanical device performance improvements often hinge on electronic drive and control technology, making price an issue.

George/Steinmeyer: It was a matter of need. In the past, when 600 in./min was deemed fast and machines sat idle for long periods of time, ball screw life of five years (400 to 500 million revs) was a reasonable design goal. Material fatigue was the only failure mode to consider. In today’s high speed machines running 2,000 in./min, 400 to 500 million revs can be reached in less than two years. As for the use of ceramic balls, it was and is an expensive alternative.

John/Amacoil: Improvements in mechanical components are also paced by the development of automated machining equipment. Name/Kerk Motion Products: And not all manufacturers have the design resources to create new products that employ the latest tools, technologies, and materials.

What can DESIGNERS do to optimize productivity?

Tom/Kerk Motion Products: Brush up on the technology. For example, many engineers still believe that a lubricated ball screw can run longer and cleaner than a well designed lead screw. Just the opposite is true. In many instances, a lead screw will last longer, run cleaner, and require no maintenance while costing 20 to 90% less.

Paul/Specialty Motions Designers should try to minimize the factors that limit the life of the screws. The screw should be accessible for maintenance, protected from debris, and sized properly.

Robert/Danaher: It all starts with the correct choice of screw or actuator. Proper screw diameter, lead, nut type, and mounting configuration are essential, but accessory choices such as support and liner load bearings can have a tremendous impact as well.

James/THK America: Know the components. Every drive system has inherent strengths and weaknesses: Ball screws can have a large degree of mechanical advantage (input torque vs. the axial force they are capable of generating), but are limited in stroke length and speed. Beltdriven units are capable of long stroke lengths and high traverse speeds, but are not appropriate for precision positioning applications. Lead screws (commonly known as Acme Screws) can be inexpensive and have high degrees of mechanical advantage, but may experience high frictional resistance. Know the nature of your application and what is truly required, then choose a drive mechanism and bearing support system that satisfies those requirements accordingly.

What can END USERS do to increase productivity?

Paul/Specialty Motions End users should set up a maintenance schedule to periodically clean off dust and debris and also to lubricate the nut. Consider the duty cycle of the screw, the harshness of the environment, and the severity of application (impact loading, speed, acceleration).

Tom/Kerk Motion Products: Simplify. With screw drives, use step motors (or motors with built-in encoders) to eliminate sensors and switches. The motor controller can accurately set rotary position, achieving high linear accuracy and repeatability with a properly matched screw. Productivity goes up as change-over time decreases and time between failure increases.

Robert/Danaher: End users should work closely with their supplier’s engineering and design personnel — whether the supplier is an OEM machine builder, a system integrator, or the component manufacturer — to be certain their requirements are well understood and that they are not seeking productivity levels that drive machine performance requirements beyond the capabilities of the rotary to linear devices in the system.

Andy/Rockford Ball Screw: Talk to component manufacturers, especially if a device is to be used in an extreme environment such as high/low temperatures, clean room, radiation, or wash down. End users also should know that all lubricants are not the same. Grease structure may be destroyed if greases of different thickeners are mixed.

John/Norco: Choose components that are easy to maintain and repair without requiring complete disassembly.

What’s new and promising for the future of rotary to linear converters?

Andy/Rockford Ball Screw: New materials such as silicon nitride balls and new types of stainless steels promise to increase life and help with harsh environments. Such improvements are necessary to keep pace with advanced control technology and ever shrinking cycle times.

John/Norco: The most valuable new ideas for linear to rotary converters will be in the development of materials and finishes that improve product durability and life.

Robert/Danaher: Improvements in lubrication will increase life and accuracy of screw-based systems. New materials, including ceramics and plastics, will result in higher load capacities, higher duty cycles, and longer life (less wear and tear). In addition, the miniaturization of both screws and motors will allow electromechanical actuation to be used in more places than ever before.

Michael/Joyce/Dayton: The two biggest enemies of screw jacks are heat and improper lubrication. New synthetic lubricants have been introduced that will substantially increase the life of screw jacks.

Wayne/SKF Motion Technologies: For ball and roller screws, hybrid designs featuring ceramic rolling elements are in the development stage with the potential objective to improve capacity and performance. Designers also are evaluating the feasibility of “self-metering” lubrication systems built into ball or roller screw assemblies, which essentially would perform this routine task automatically and help promote performance life.

John/Exlar: Advances in manufacturing and heat-treating continue to yield incremental improvements. However, some “improvements” are little more than gimmicks that strengthen one area at the expense of another. Poor quality materials are also finding their way into components to save cost, sacrificing performance as well as productivity.

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