#Industry News
How to Optimise Round Rail Performance and Cost with Informed Linear Bearing Selection
When designing for linear motion systems, the initial decision between round and square rail can significantly impact cost, performance, and durability.
Square rail, which is typically the more expensive option, has a higher load capacity and tighter accuracy. Round rail offers distinct cost and reliability advantages for certain applications, but maximising those advantages depends on specifying the right bearing to minimise friction in each application. Designers must often choose between plain bearings, often called bushings, and ball bearings. And within each type, they will have more options that impact price, performance, maintainability, and durability. Understanding these options early in the design cycle will optimise the performance and cost of the linear motion application.
Plain and simple
Because plain bearings have no rolling elements like ball bearings, they are the least expensive to manufacture and thus cost less. They also outperform assemblies with rolling elements in terms of smoothness and quietness, and because they have fewer moving parts, they are less susceptible to dust and dirt.
Plain bearings with elastomer liners amplify these benefits. They are available with high-performance, bearing-grade materials applied to aluminium or other substrates to provide good load capacity (20% of that of a ball bearing) and a reasonably low friction coefficient, typically between 0.05 to 0.25.
Since the contact area of a plain bearing is distributed over a large area, plain bearings can also be used with unhardened round shafting, giving designers more flexibility to choose materials that address other design requirements such as corrosion protection or weight.
Some manufacturers make plain bearings from self-lubricating polymers that minimise the need for maintenance. (Figure 1)
The cost savings and smooth operation of plain bearings, however, come at the cost of load handling, precision and speed. They are best for applications in which alignment and moment load are non-critical or those that require shafting that is incompatible with traditional Ball Bushings® such as aluminium or 316 stainless steel. Plain bearings typically handle static loads up to 50 kN (11,000 lbf), and dynamic loads up to 9 kN (2023 lbf), and speeds of up to 0.5 m/s (100 fpm). When those parameters are met, plain bearings -- especially those using polymers -- are beneficial in both clean and harsh environments. Primary applications involving light loads, lower speed and precision are found in packaging, factory automation, robotics, health and fitness, woodworking, food and beverage, and vending. Applications that might also involve ultra-clean, toxic, or harsh environments include semiconductor production, medical automation, and food production.
Scaling up
Applications requiring higher load handling, speed or precision will need bearings with internal rolling elements. The type of bearing specified will vary according to the number of balls used, directly impacting the allowable load and speed.
The precision steel design (Figure 2a) uses hardened steel linear ball bearings, which have point-to-point contact on a fixed point. This enables the handling of dynamic loads up to 22 kN (5000 lbf) and diameters from 6 to 100 mm.
The next performance level up embeds a single ball track in a conforming groove on a floating plate. (Figure 2b) This bearing has a low coefficient of friction and can triple the load handling capability over the fixed plate and increases bearing life up to 27 times longer than precision bearings. This configuration is optimal for rail 5 to 50 mm in diameter and dynamic loads up to 13 kN (3000 lbf).
The more load-bearing balls in the load zone, the greater the load handling and durability. Deploying two ball tracks in a conforming floating track, for example, can increase the load capacity six times and bearing life 216 times over the precision bearing. (Figure 2c) This configuration is also optimal for rail 12 to 38 mm in diameter and dynamic loads up to 17 kN (3880 lbf).
Self-aligning bearings extend life
When high load handling, speed and precision are high priorities, a self-aligning bearing can maintain a low coefficient of friction while maximising bearing life. Some manufacturers accomplish this by designing the bearing plate so that the radius of its outer surface is smaller than that of the inside radius of the hardened precision ring. This allows the bearing plate to roll against the ring, evenly distributing the load on each of its two ball tracks. The bearing designs also allow the bearing plates to rock 0.5° about the hardened precision ring to ensure smooth ball entry and exit in the load-carrying zone, and optimum ball loading from end to end.
This self-aligning structure compensates for the torsional misalignment that might result from inaccuracies in base machining or machine deflection, with little increase in stress to the bearing components. Once a self-aligning bearing is mounted in an adjustable housing, designers can achieve a selected preload. The bearing design allows the bearing plates to move radially, which provides the ability to achieve close bearing/linear race fit-ups for high-precision applications.
Furthermore, although ball bearings are not necessarily as environmentally resistant as polymer plain bearings, certain actions can be taken to protect their high load handling and durability in corrosive environments. This includes specifying stainless steel, nylon, or nylon-coated rolling elements in corrosive environments.
In closing
If applications requiring low cost, smooth and quiet operation, or reliable performance in corrosive environments are more critical to your operation than high load handling, precision or speed, you should consider a plain bearing. If your shafting requirements are incompatible with traditional ball bearing materials such as aluminium or 316 stainless steel, polymer materials should be considered. If durability and maintainability are most important to your design, self-lubricating plain bearings would best suit your needs.
Otherwise, you will need a ball bearing, in which price, precision and load handling will vary depending upon the number of balls loaded, whether the ball track is fixed or floating, or whether the assembly is self-aligning. We have presented general guides on bearing selection, but this, of course, will vary according to your application’s requirements. Resources such as the LinearMotioneering® sizing and selection tools on the Thomson Industries website (www.thomsonlinear.com) can help you optimise bearing selection for your round rail applications.
