Productivity Forum: Linear Slides & Bearings

April 1, 2003
Wondering what's new in the linear world? The editors of Motion System Design conducted a survey among industry experts to find out. Here are the responses we believe you’ll find most useful

When using linear slides & bearings, what applications present the most challenges in terms of machine productivity and why?

BEN/POLYGON: The role becomes most critical in an environment where high levels of contamination exist. Applications where the environment may be very dirty require a design review that takes into account pin selection, liner construction, and seal design. Excessive debris that is not properly managed will result in premature shaft scoring, will attack the integrity of the bearing system, and will drive the wear of the bearing, as well as destroy any desired frictional response between bearing and shaft.
Environments where lubrication cannot be used. Most every type of bearing works better with proper lubrication. However, certain applications, such as biomedical equipment, cannot tolerate the presence of oil near sensitive processes. Also, applications which serve in the presence of corrosive chemicals. Materials such as acids or strong base chemicals can etch away material from the smooth surfaces of the wheels or tracks, thereby accelerating the wear of the aforementioned components.
The most demanding application in terms of productivity today is in semiconductor processing, where operating cycle times are contained, in some applications, in fractions of a second. In such applications, the acceleration requirements and the dynamic behavior of the rolling element systems are stretching the limit of the current technology. The most demanding applications in terms of durability are in the woodworking and processing industries where the environment is such that the linear units employed are limited by the duration of the lubricant.

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What are the worst cases of improper design and implementation you’ve seen? Describe what went wrong and how these and similar problems can be prevented.

ANDREW/ROLLON: We often see cases where the bearing chosen is much too precise for the application. This can create some serious problems. Precision bearings must be mounted precisely. Designers must choose the right bearing for the application. Are you moving the head of a machine tool? Then you must use a highly precise and rigid bearing. Are you making a pick and place assembly machine? Use a bearing that offers precision as will as mounting forgiveness.
Many application designs assume that the intimate contact between bearing and shaft will be sufficient to “wipe” the debris out. This proves to be very inaccurate as the bearing is eaten away and the pin begins to show heavy scoring. Shaft material problems are almost always related to one of two issues: selecting a pin that is too soft or selecting a pin that does not have the appropriate treatment to limit the shaft’s corrosion. In both cases, a better understanding of the application environment and flexibility to change shaft materials would have allowed the design to function without a similar problem.
BOB/BISHOP: Usually bad designs are based on improper engineering analysis of loads and forces applied to the slide. Motion system designers must be careful to consider all of the forces produced during the process, some of which are not always obvious.
The worst cases that we’ve seen have resulted from severe undersizing of the linear bearings used. This can and will cause the linear guides and bearings to reach their fatigue life much sooner than the machine designer expects. Problems occur when application parameters are not thoroughly considered during the design process.
The customer used recirculating ball systems to support trolleys. The rails were bolted to a welded structure, not machined. The bearing units were experiencing erratic friction (due to lack of localized alignment) and occasional failure. We replaced the design with large track roller systems that offered both a lower and more uniform friction as well as an increase in maintainability (the rollers could be replaced without taking any section of the machine apart) with a great improvement in uptime.

Sometimes the simplest design is the one that works best. More sophisticated linear systems, while they offer many theoretical improvements, may have drawbacks that no one is willing to discuss.
Design error can cause catastrophic failure of equipment. If the application calls for short-stoke performance, the wrong bearing choice can seize on the shaft. This leads to shaft or rail damage, and can even shutdown the motor or drive mechanism.

What are “best practices” when designing with linear slides & bearings?

ANDREW/ROLLON: The best advice is to understand the dynamics of the application and remember your simple high school physics. How much load is on the bearing? How fast must it move? Are there accelerations or are there other actions going on which would effect the bearing? Where is the load in respect to the bearings? How is the environment? What is the expected lifetime? How much mounting space is available?
The best practices when designing linear slides and bearings requires a design approach that looks at five factors: the velocity of the slide’s travel, the rate of de-acceleration, the required precision of the system, the pin material, and the level of contamination.
“It’s a light load,” or “It’s not going to go very fast,” lacks the required technical information. This ambiguity can lead to an underestimation of the application, which in turn may result in improper sizing of the slide system. Invest in the extra time to document what you plan to do, and the provider will be able to recommend the right product.
Engineers should size bearings for their application by using no more than 15% of the suggested dynamic load capacity of the bearing. This ensures that adequate load reserves remain as a safety factor for unknowns.
The bearing durability is affected by the effectiveness of its protection and of its lubrication. Hence sealing design (woodworking equipment, machine tools, food packaging) is critical. Lubrication is also an essential part of the design process. Linear bearings do not require a large volume of lubricant to operate properly. However, they require the correct lubricant. Using large volumes of lubricant, especially of the wrong kind, may prove detrimental.
When designing for linear motion applications, research among different companies and products. Determine what is the motivating factor in the design (price, performance, precision, delivery, vendor reduction) and then seek out the product and company that is the best fit.

For self-lubricating linear bearings and slides, the most important issue to address with potential customers is the 2:1 rule. This rule determines the maximum allowable distance for mass and drive forces when using selflubricating bearings. If this 2:1 rule is exceeded, binding may occur in the linear system.

What can linear slide & bearing manufacturers do to improve productivity?

ANDREW/ROLLON: Linear bearing manufacturers can listen more to the customers’ needs. We believe that there is no reason why linear bearings can’t exceed the expectations of our customers.
It is critical to drive a genuine discussion and mutual understanding between vendor and customer as to whether the target is the lowest system cost or the lowest total cost of ownership. Identifying the interplay between cost and productivity has to occur early and honestly for a successful design partnership.
Continuous product innovation, spurred by competition among engineering-focused linear motion manufacturers, results in constant productivity improvements. Machine builders need to provide as much value as possible to their customers, which in turn creates the top priorities for our R&D departments.
RUSS/INA: Manufacturers are working to continuously increase the operating limits of their products. Linear bearings can now operate with linear velocities approaching 10 m/s and accelerations in excess of 10 g. The linear systems of today require less maintenance. Systems capable of operating in difficult environments with long maintenance intervals are available off-the-shelf today from many companies. The improvements in sealing design and lubricant compositions have helped to make this possible.
Linear bearings can allow machines to operate faster, which increases the number of units produced by a manufacturer. Polymer bearings that can operate in harsh contaminants reduce the possibility of machine failures, and the need for preventative maintenance.Manufacturers can provide machined shafts, and rails cut to length free of charge.

How do choices made involving linear slides & bearings affect other areas of the machine or system?

ANDREW/ROLLON: Smaller, more compact bearings allow the machine to fit into a smaller footprint. Easily mounted or self-aligning bearings can be mounted to structures assembled from aluminum profiles without fear of binding due to parallelism. Linear bearings with telescopic movements can allow smaller machine frames to be built. B
Noise is sometimes a consideration. Fast moving linear slides can generate excessive noise, seriously affecting the customers’ perception of the system quality.

MTBF (mean time before failure), and MTTR (mean time to repair) are often important criteria in maintaining system productivity. Verify the availability of slides and replacement parts when specifying linear components.

Friction can play an important role in system life and efficiency. Certain linear components must be tightly sealed to exclude contaminants, which in turn wear out the seals prematurely or produce heat and excessive friction.
Type and size choices made in linear guidance units can affect things like motor size, output quality, machine throughput, and total cost of the machine.

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