Castor Wheel Flutter is a phenomenon whereby a Castor Wheel attached to a piece of equipment, for example a Trolley, shakes or rotates uncontrollably from side to side.
Many people will have experienced Castor Flutter at the supermarket. Ever noticed how one of the wheels of the Shopping Trolley seems to be rotating of its own accords? Yes!, well that’s called Castor Wheel Flutter.
Castor Wheel Flutter is caused by harmonic excitation of the castor wheel. Simply put, when the wheel tread deforms and rebounds, due to force and movement, oscillations are caused. Rubber Castors or Polyurethane Casters would be a prime example of high deformation wheels, in which we will use for the examples within this article.
In order to understand how these “Oscillations” cause flutter, we first need to understand each stage of the wheels oscillation. It’s a relatively simple 2 stage process, as follows:
These are the 2 stages that occur as the castor wheel is travelling. Each Time Point 1 and Point 2 occurs, it is considered 1 oscillation. In the above example, one oscillation occurs every 100 milliseconds (or every 0.1 seconds), therefore it can be expected that the wheels are oscillating hundreds, if not thousands, of times within a short distance of travel.
Castor Flutter is due to the “time phase lag” between the “initial deformation force” and the “reactive rebound force”. In the above example these two forces lag each other by 100 milliseconds (0.2 Seconds). Each time the rebound force is generated, it tries to re-centre the wheel, but due to the time phase lag, the force over corrects and swings past the centre position. The next oscillation then over corrects in the other direction, causing the wheel to swivel from side to side. This action repeats itself over and over.
The Speed at which these oscillations occur is known as the Disturbing Frequency (i.e. the frequency at which something occurs due to outside influence). At relatively slow speeds, for example 2 to 3 MPH, the disturbing frequency and resultant correction forces are low, allowing the Castor Wheels friction (surface friction and Bearing Friction) to control the correction, however with increasing speed the disturbing frequency and force increase to the point where the wheel friction can no longer control the forces, and Castor Wheel Flutter occurs. (This may also be known as Castor Wheel Resonance, where the Disturbing Frequency and the Castors own Natural Frequency is within 44% of each other)
Considering the above the 2 primary factors that cause Castor Wheel Flutter are “Speed of the Wheel” and “Weight upon the Wheel”.
For small appliances, or equipment travelling at slow speed Castor Flutter is not an issue. However there are many applications where Castor Flutter can have significant consequences.
To give an example, consider a warehouse trolley in a busy distribution centre. In order to be efficient, the operators must work at an efficient pace, meaning that they may be pushing or pulling the trolleys at a relatively fast pace. If one of the Castor Wheels was to flutter, the operator may lose control of the trolley. This could result in colliding with an object, running over a person’s foot, or even tipping of the trolley. This could have a serious impact on both the commercial and liability responsibilities of the warehouse owner.
Castor Flutter can cause Loss of Control, Reduced Handling, Wear of Bearings and Wheel Tread, and Excessive Noise.
It should be noted that different applications may require different levels of control. From Shopping Trolleys to Aerospace and Luggage Handling Equipment, the consequences of castor flutter are different. For non-critical applications a standard commercial Castor Wheel should give satisfactory performance, whereas for critical applications a highly controlled designed Castor may be preferred, with high tolerance, machined race ways and regular lubrication maintenance.
The most obvious answer to stop Castor Flutter completely is to either reduce the Speed or reduce the Weight applied to the Castor (or both). However from a practical, commercial & efficiency perspective this may be difficult to achieve.
It is therefore normal practice to “Control” the Castor Flutter, rather than “Eliminate” it, and it should be expected that all Castor Wheels will “flutter” at a given speed and force.
Below, we will discuss some of the ways in which Castor Wheel Flutter can be controlled:
You can view a demonstration video below (including Slow Motion):
Here at Bulldog Castors Ltd we like to set ourselves apart from the rest. This is why we focus heavily on the many aspects of Engineering & Design to ensure that our products meet the exacting standards that our customers require.
We employee specialists in Vibration and Shock Control that you will not find with any other Castor Wheel Manufacturer. When it comes to castor flutter, we have the tools and knowledge to ensure that your product is performing to its best.
Whether you are experiencing Castor Flutter, or you are designing a new application, the Team at Bulldog are happy to help. Contact them today at sales@bulldogcastors.co.uk or +44 (0)116 2970521
