Manufacturers use different methods to calculate load ratings of linear motion products, which can affect how well they perform in an application. To make informed decisions, pay close attention to details, ask hard questions of suppliers, and read the fine print. By understanding the varying methods of calculating load ratings for linear guides, you can find the most suitable product for your application.

In the field of linear motion, the life calculation of the product is directly impacted by its load rating. In general, the higher the load rating, the longer the theoretical life will be. If the load rating is artificially inflated or calculated without using comparable standards, it may lead to specifying a product that, in real-life applications, may not be able to reach the goal it is designed to achieve. Catalog load ratings among linear guide manufacturers vary by as much as 50% for the same size bearing block.

Standards are established to counter this detrimental tendency and provide a clear and consistent baseline for comparison. In the manufacturing world, adherence to International Standards Organization (ISO) standards helps to ensure that users are really comparing apples to apples when evaluating products. However, the usefulness of these formulas can be minimalized when manufacturers provide ratings that are calculated using modified ISO standards or other means. In the area of linear guides, this can lead to an advantage on paper that may be impossible to attain outside of a laboratory.

In the field of linear motion, the life calculation of the product is directly impacted by its load rating.

ISO standards were established in 2004 for determining the dynamic and static load ratings for linear motion rolling bearings. The applicable standards are ISO 14728-1 and ISO 14728-2. While these standards are fairly straightforward, the industry continues to see published load ratings that have been obtained through alternative methods.

In any ball linkage retaining system, either the number or size of the balls must be reduced in order to make room for the linkage mechanism, be it plastic chain or resin spacers.

For load ratings that claim to be based on ISO standards, investigate how the numbers were obtained. When used as intended, companies input relevant information into formulas such as the one shown above. That number is then used in a separate ISO formula to determine estimated product life. Typically, the manufacturer then tests a large statistical sample of the product at varying loads and speeds until the product fails, ensuring adherence to the ISO standard load rating and life calculation.

One alternative practice involves backward calculations of load ratings from testing a small quantity of product to failure. Under a backward calculation method, a set number of linear motion rolling bearings are run to failure in a laboratory. These trials are used to determine an average product life. That number is plugged into the ISO formula for estimated product life and a load rating is determined. But this method creates problems on several levels. First, it greatly reduces any safety cushion that may have been provided by a larger statistical sampling. Second, because the tests are carried out in a carefully controlled environment, the resulting expected product life is artificially inflated when compared to performance in actual operation conditions. Lastly, ratings obtained through backward calculations can be up to 1.5 times more than what would be generated using the ISO formula.

Another source of confusion occurs when linear guide manufacturers add a positive life factor for using a ball linkage retaining system within their design. One such claim is that a ball linkage system can increase load ratings and life simply because the linkage retains lubrication. However, this factor has been rejected by ISO. There is no factor in ISO formulas for lubrication. Thus, the ISO load rating formulas already assume lubrication is adequate. Therefore, any lubrication system can only help reach the dynamic load rating or life as defined in the ISO formulas. Yet, some manufacturers use it, arguing that ball guides incorporating ball-linkage retainers improve load ratings. In certain circumstances, ball linkage retaining guides are better options. The minimization of ball-to-ball contact offers relative smoother and quieter operation. At high load levels of 0.35C, where C equals dynamic load capacity, some initial data indicates a possible benefit in product life from ball separation. At the same time, the vast majority of applications run at load levels of 0.10C to 0.15C, a range where no potential advantage in product life has been detected. You should not assume that an increase in product life at high load levels justifies an upward adjustment of load ratings across the board.

Load ratings are understood as a function of ball size, contact angle, and the number of balls carrying a load. In any ball linkage retaining system, either the number or the size of the balls must be reduced to make room for the linkage mechanism, be it plastic chain or resin spacers. Sometimes, in order to recover from this loss of space, the bearing block is lengthened, resulting in lost stroke and increased envelope.

Ball linkage retaining systems also can result in lower margin for error, increased sensitivity to contamination, and decreased maximum moment loads. Thus, it can be misleading that some manufacturers add a ball linkage multiplier when determining load ratings rather than reducing them. This will lead to a theoretical life advantage that is inflated beyond the product’s true capability.

Some manufacturers using linear guides with ball linkage retaining systems take another approach to stating their products’ specifications. In some cases, it is openly admitted that this type of design will reduce the load capacity of a linear guide. Other manufacturers will forego ISO standards and calculate load ratings by using the Deutsches Institut für Normung (DIN) standard multiplied by a set percentage. When facing such a situation, it is an apples-to-oranges to figures obtained from the ISO standard. A ball-linkage retaining system may provide benefit in some circumstances, but you must fully understand a manufacturer’s test data and apply that information to your specific application.

Editor’s note:  This article represents Mr. Williams’ experience with load ratings in linear systems. If you have an opinion on this subject, please go to the Engineering Exchange at www.engineeringexchange.com.