Bearings and slides

March 1, 2008
The basics How do bearings and slides contribute to productivity in today's motion-centric automation environment? Matt igus: An important development

The basics

How do bearings and slides contribute to productivity in today's motion-centric automation environment?

Matt • igus: An important development in recent times has been the emergence of self-lubricating polymer bearings. These eliminate the need for maintenance involving messy oils and help reduce machine downtime. Solid lubricants embedded in fiber-reinforced bearing material transfer to the surface of the shaft during movement. Sliding bearings also achieve faster cycle times because they're not affected by acceleration, as are ball bearings.

Mark • Rollon: With much of the focus for today's multi-axis, motion-centric integrated systems rightfully being placed on sensors, motors, and controllers, often times the mechanical portion of a design is overlooked. In reality, the system will only be as good as the weakest link in the design.

Selecting a bearing or linear actuator on little more than convention or brand can end with mediocre results, limiting productivity or even causing premature failure. Investing the time, however, to analyze and select optimal bearings or guides will result in greater productivity than anticipated, increased uptime, and greater life expectancy.

Dice • THK: Because today's industrial world is becoming more complex and sophisticated, the need for high performance linear motion devices is apparent. Whether the application is associated with the electronic and semiconductor industry, factory and office automation, precision instrumentation and robotics, or even space/aeronautics, the demands for better productivity of automation industries are ever increasing.

Some of the expected qualities of linear motion guides include faster travel speed, better positioning accuracy/repeatability, stronger and higher load carrying capacity, long-term maintenance-free operation, on-time delivery, substantial energy savings, quiet operation, and low total cost. This calls for a guide that offers: smooth movement with minimal or no clearance, precise running accuracy, long life, superior positioning accuracy, high rigidity in all directions, large permissible load rating, and high-speed operation.

Tried-and-true tips

What's your best advice on specifying, sizing, and applying bearings and slides where productivity is the goal?

Matt • igus:

  • Specify bearings that need no lubrication or maintenance. This eliminates downtime, increasing machine productivity.
  • Select the right bearing material for the application. Will the bearing perform under water, support high loads, or work in dirty environments?
  • Select the right shaft material. Many bearings last longer with a rougher shaft.
  • Specify a bearing with a predictable wear rate and service life.
  • Choose a bearing that does not require reaming or honing before use.
  • Specify a split bearing, which can be replaced without removing the entire shaft.

Mark • Rollon: Don't cut corners. Take the time to investigate options, learn about new technology, and review new products and designs.

Gather ALL application specifications before making a product selection, such as load, center of gravity, moment loads, speeds, and cycles needed.

Consider hard to quantify factors such as environment, access for maintenance, and how equipment is to be shipped.

Dice • THK: When deciding which type of linear guide is most suitable for a particular application, there are many design issues to consider:

  • Available space, required stroke, mounting orientation
  • Loading scheme (minimum safety factor and rigidity)
  • Drive method (screw, belt, hydraulic, pneumatic, linear motor, etc.)
  • Motion profile (maximum velocity/acceleration)
  • Accuracy requirement and duty cycle
  • Desired service life (time or distance)
  • Environmental conditions and contamination protection
  • Lubrication and surface treatments
  • Materials (steels, ceramics, plastics, etc.)

To maximize productivity for a particular application, follow these steps:

  1. Define service conditions — set the conditions for the design of loads on the linear guideway.
  2. Calculate the applied load — calculate load exerted on the carriage.
  3. Calculate static safety factor — verify the value of the static safety factor for the basic static-load rating and maximum applied load.
  4. Calculate the mean load — convert fluctuating applied load into a mean load.
  5. Calculate the nominal life — calculate running distance and service life.
  6. Predict the rigidity.

Yugi • IKO: Be sure to use the proper type and size of linear bearings to achieve the necessary bearing/machine life, accuracy, and rigidity. Using high quality bearings will result in adequate frictional resistance and smooth motion. Doing sufficient performance confirmation tests of a machine in the design/prototyping stage may also be recommended, especially in cases where the application has a smaller safety factor bearing with smaller or fewer bearings for cost reduction or machine downsizing reasons.

Worst-case scenarios

What's the worst that can happen if a bearing or slide is not specified or installed correctly?

Mark • Rollon: An engineer's nightmare is to have specified a bearing or actuator and then have it fail shortly after installation. We were called in recently to help solve just this situation at a large energy drink canning facility. The engineer called looking for answers after he had installed a linear actuator from another company that failed in two weeks.

After analyzing all of the factors involved, a moment load was discovered that had not been calculated into the original design. A pusher that moved a stack of lids along a chute 10 in. away from the actuator created a resulting 300 in-lb. moment. Once the entire design picture was in focus, the correct actuator could be specified. The key to finding the right solution was having the actual application parameters and then choosing a linear system that could handle them.

Dice • THK: In general, selecting a linear motion guide that is not appropriate for an application or is installed incorrectly can decrease productivity. Selecting the wrong guide for a harsh environment such as woodworking or in a clean room where dust generation is a problem will cause abnormal wear or shorter service life. It's important to select an effective sealing or dust-control device that addresses the environmental conditions. In environments where corrosion resistance is required, a stainless steel linear motion guide is necessary. In high-speed applications, optimum lubrication is necessary to reduce heat generation and the wear rate of rolling elements and raceways.

Matt • igus: Ball bearings do not function as well as sliding bearings in dirty environments and can often damage shafts. The problem is that abrasive dirt particles prevent balls from rotating, causing them to score the shaft. The advantage with sliding bearings is that, even if the shaft becomes dented or scratched, they are not affected. However, if sliding bearings have not been fixed properly into their housing, they can creep out when subjected to high temperatures or high accelerations.

Yugi • IKO: Lack of lubrication maintenance often leads to unexpectedly short bearing life. Worse yet, if the bearing failure impacts other parts of the machine, this can cause even longer and more serious equipment downtime.

Future perfect

If you could create the ideal bearing or slide, what would it look like? What would it do?

Matt • igus: I would invent a plastic bearing with an ultra-low coefficient of friction, less than 0.05, which would enable additional cantilevered loads in linear applications.

Mark • Rollon: Until stardate 1312.4 rolls around and we can move things from point A to point B with a transporter, we will have to deal with solid tangible material. Due to the broad range of applications in automation, the ideal bearing or linear actuator needs to be one that is versatile and could be adapted to meet a variety of application needs.

Yugi • IKO: Bearings that offer excellent load capacity and very small frictional coefficients will allow for smaller driving devices, which will permit significant downsizing of machinery and, as a result, less energy usage.

Industry expertise

Matt Mowry
igus Inc.
(800) 521-2747

Mark Huebner
Rollon Corp.
(877) 976-5566

Dice Kanamatsu
THK America Inc.
(800) 763-5459

Yugi Ikeuchi
IKO Int'l. Inc.
(310) 609-3988

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