What is a Safety Factor (SF) for casters, how do I improve ours, and other important safety-related questions.
Q:What is a Safety Factor (SF) for casters
A:Safety factors help us calculate load and speed ratings due to the variabilities in caster stress.
Safety factors (SF) exist in engineering because of our inability to determine or predict the actual forces, speeds, or other parameters for a given application. For casters, uneven riding surface, variable speeds, changing turning radius, and changes in load distribution all play a part in the variability of the stresses a caster undergoes during use. So it is prudent to use a safety factor when calculating load and speed ratings for casters. ??
The required safety factor can vary depending on certain industries’ standards. For example, in aerospace and military applications, customers typically require a 3X safety factor based on the material’s yield stress or a 5X safety factor on the ultimate tensile stress of that material, or whichever is higher. The standards are slightly less strict in some automotive applications, with a 2X on yield and 3.5X on UTS requirement.
Huanxin develops many of our caster wheel products for specific industries, so our SF standards will vary. However, typically we provide load ratings that have a 1.5X SF on functionality, 2X SF on yield, and a 4X SF on UTS. This means our casters remain functional when subjected to 1.5X the catalog load rating. At 2X the catalog load rating, components on the casters will begin to deform, and then at 4X the catalog load rating, expect the caster to start to see catastrophic failure.?
Higher SF typically is associated with higher costs. So a higher SF may not always be desirable. The optimal goal is to understand the application variations and ensure that the SF is large enough to accommodate those changes. As always, it is a good idea to talk to Caster Concepts engineers to obtain specific SF on a particular product.
Common caster failure causes.
Failure of a caster comes in many forms. At higher speeds, the weakest link is the polyurethane treads. Swivel bearings and wheel bearings typically wear out first at high shock loads. At high static load, caster legs — the pieces of structure which support the wheels — or the wheel axle begin to deform. There are always ways to improve strength depending on failure modes. One way is to utilize higher-performing polyurethanes that do not generate as much heat at a given load and speed. Larger, more shock-resistant bearings can also be used in the design to provide longer life. Using additional material can also reinforce structural members under high stress.
10 reasons castors fail and how to avoid them
1. Extreme temperatures
Industrial castors used at temperatures exceeding 1000F can cause the material to fail. Softer materials like polymers and rubbers can flat spot or even melt.
Avoid by using temperature-resistant materials, such as hard-wearing phenolic resin or high-temperature rubber wheels. These materials can typically stand up to temperatures ranging from -40° to +280°.
Time is money, but running castors at speeds they’re not designed for can result in detrimental impact forces and overheating.
Avoid by finding out the speed at which the castor will be used in your application and selecting a castor with a speed capacity that’s up to the job.
All castors have a capacity rating. If the load capacity exceeds this rating, the castors will fail.
Avoid by calculating the load capacity for each wheel. Simply divide the combined weight of equipment and maximum load by the number of castors to be used. (Learn more here. ) Then to be on the safe side, choose a castor with a higher capacity than your calculation to allow for shock loads, rocking or poor floors.
4. Uneven loading
If one castor is made to carry a significantly higher proportion of the load, it can lead to premature wear and complete failure.
Avoid by taking the time to understand the load distribution of the castors and apply this to the castor design.
5. Impact loading
This is when a castor hits a large obstacle and experiences the associated g-forces, causing the castor to bear a load higher than its capacity rating.
Avoid by considering the use of spring-loaded or shock-absorbing castors.
6. Wrong bearings
One bearing type does not suit all. Choosing a type that’s not suited to an application experiencing extreme temperatures, humidity, liquids, corrosives or impacts will increase the risk of premature castor failure.
Avoid by matching the right bearing to the appropriate castor for your application, taking into consideration bearing material, shape and accessibility for maintenance.
In addition to extreme hot and cold temperatures, wet environments, corrosives and debris can take their toll on certain castor materials – where they’re well suited to some but not others.
Avoid by knowing what’s in your application’s environment and then specifically selecting the castor material that’s best suited.
8. Excess swivel offset
A design flaw can result in too big a distance between the centre of the axel and the centre of the kingpin or main rivet. This could cause the legs of the yoke to break away from the swivel when a load is applied.
Avoid by ensuring the design and engineering teams collaborate to find the optimum balance between a large enough offset for ergonomic efficiency and a short enough offset to ensure joint strength.
Rough terrain such as rough, uneven or sloping surfaces can shorten the lifespan of a castor, particularly soft wheels.
Avoid by ensuring the castor has a shock-absorbing element to it.
This is the wear that is pressed by the ball bearings in the swivel head into the hard cap. It affects the performance of the swivel of the castor by increasing the swivel force.
Avoid by using the Brinell hardness test as part of your calculations to ensure you use the right materials.