Polyurethane Applications in Heavy Duty Industrial Casters and Heavy Duty Industrial Wheels
Caster flutter is a phenomenon in which a swivel caster is harmonically excited such that it begins to swing uncontrollably from side to side as the wheel rolls forward. Caster flutter results in excessive noise, increased wear on all bearing elements and loss of tracking and control of the dolly or cart in which the caster is installed. There have been numerous studies to understand the causes of caster flutter. The three most common causes of caster flutter in industrial casters and wheels are:
- Sloppiness in Swivel Section - If the swivel section is too loose, the caster is more likely to flutter. A loose swivel section no matter the diameter of the raceway will establish as resonance leading to flutter.
- Misalignment of Caster to Ground - If the caster is either mounted incorrectly or the caster legs have been bent, a resulting misalignment will cause the caster to flutter.
- High Speed – It is important to note that all casters will flutter once it hits a certain velocity and is excited at its natural frequency. In most applications involving industrial casters and wheels the wheel is not balance and lends itself to fluttering.
There are a number of techniques that can be employed to minimize caster flutter. While all the listed options my not work in your particular application you may find some that are helpful to correct your problem with fluttering.
- Increase swivel lead – A longer swivel lead will reduce the forces that push the caster sideways. It will also increase the caster’s natural frequency and as a result the caster will only flutter at high floor speeds.
- Precise Bearing Swivel Section – Though a more costly option, casters with precision machined ball raceways flutter less than those whose raceways are manufacturing through a stamping process. The precision bearing , when manufactured properly will dampen the oscillation and prevent fluttering.
- Heavier or Thicker Grease - Heavier grease will damp out the oscillation much like the damping in your shocks in your automobile.
- Reduce the Mass of the Wheel - Just as increasing swivel lead increases the natural frequency of the caster, reducing the mass of the wheel will also increase the natural frequency of the caster as well. As a result, the caster will not flutter until it reaches a higher speed.
- Reduce Velocity – The speed at which a caster travels is the main cause of caster flutter. At low speeds, 2 or 3 MPH, a swivel caster may behave quite well. However, at higher speeds, 8-10 MPH, that same caster may flutter uncontrollably.
Change Poly Composition to Increase Friction with Ground Increasing the friction with the ground has a small effect on reducing flutter. Like thicker grease, higher friction acts as a means to dampen out the oscillation reduce the chance of building resonance.
The engineering staff at Caster Concepts, Inc has extensive knowledge of fluttering issues of industrial casters. When you are struggling to correct that problem please feel free to contact this talented groups of caster experts to assist in the solution or to specify a caster that will not flutter in your prescribed application.
At Caster Concepts, Inc we will work with you to design the drive wheel for your friction drive application. Caster Concepts, Inc. is an ISO 9001-2000 certified designer and manufacturer of a comprehensive line of industrial casters, ranging from a medium-duty series to a heavy-duty series to meet any application. CCI also manufactures caster wheels to withstand impact, corrosion, temperature extremes and chemicals found in harsh industrial environments. These multi-functional casters and wheels serve a variety of applications in the industrial, automotive, food service, aerospace, material handling and other demanding markets.
There are many places where polyurethane products are used in industrial applications. There are many places where polyurethane products are used ie-determined curing agent that generate a thermo set material. The most common usage of urethane in an industrial application is in the covering (tire) of an industrial wheel. The purpose for the urethane use is
1. Sound deadening. The softer tire (as compared to the floor or to the steel cart) serves as a mechanism to absorb shock/vibration during the normal operation of the cart or vehicle.
2. Floor protection. The polyurethane also provides the ”insulation” of the hard cast iron wheel hub with impacting with a cement floor. If these material are allowed to interface there will be either a fracture of the grey iron wheel or the cracking of the cement flooring. If one those materials do not fail most likely there will be failure in the cart or load that is riding on the caster.
Polyurethane materials that are used for industrial wheel applications have a variety of properties that make them appropriate for the application. The two most important properties are
1. Tear Strength -Tear strength is the force necessary to tear a material, measured by the force necessary to start or continue a tear. High Tear strengths are found in applications where it would be detrimental for the tire to ”chunk out” while under load.
2. Tensile Strength-Tensile strength is indicated by the maxima of a stress-strain curve and, in general, indicates the point where the material will not return to its original form. It layman terms in how much the material will stretch prior to failing. When under load it is very important that the polyurethane material is able to stretch while not functionally or physically losing its physical make up.
3. Favorable Hysteresis Curve- Polyurethane fail in dynamic applications due to the build up of heat. Heat is generated in the soft and hard segments of the chemical structure when they are under load. Using the Tan Delta of the material a hysteresis curve can be defined for the material that can be identified for difficult applications.
Polyurethane tires also come in a variety of durometers. Durometer is one of several that measures of the hardness of a material. Hardness may be defined as a material’s resistance to permanent indentation. In the application of industrial wheels standard durometers range from 70 shore A, which slightly harder than typical rubber materials, to 70 shore D, which is as hard as a bowling ball.
The use of polyurethanes materials in the industrial caster and wheel applications has been a significant improvement over the past 50 years. It has led to improved working conditions for many employees and improved the safety for everyone. The proper use of these materials in the applications takes knowledge of the application (weight, speed, and environment) and knowledge of the physical properties of the available materials. Matching of one to another will lead to a product that provides a very acceptable life cycle.
The engineering staff at Caster Concepts, Inc. and Reaction Industries, LLC have the knowledge, the tools and the experience to assure that you are getting the most cost effective material for the application.