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:
Firstly, the Rubber Wheel deforms due to the weight and force that is applied to the equipment. For arguments sake, let’s say the wheel takes 50 milliseconds to maximum deformation.
Secondly, the Rubber Wheel recovers (rebounds) from the deformation. Again, for argument sake this takes 50 milliseconds to recover.
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.
So, how does this cause Flutter?
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”.
Why do you need to consider Castor Wheel Flutter?
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.
How to control a Castor Wheels Flutter
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:
Swivel Ball Race Tolerance & Fit – If the Ball Bearings in the Swivel mechanism are sloppy (i.e. have a loose fit), it will considerably reduce the friction of the swivel mechanism. Less Friction = Less Resistance to Swivel. If the Ball Race Swivel has a good Fit, the over-correction forces caused by the oscillations will be dampened by friction. If the Castor Wheel has been manufactured to a low standard, or where the castor bearings have worn, resulting in a “sloppy” fit, this will increase the likelihood of Castor Wheel Flutter.
Lubricating Grease – The Viscosity of the Grease on the Swivel Bearings affects the dampening effect of the forces. If the Grease is of High Viscosity it will dampen the forces quicker, resulting in energy loss, and reduced correction forces.
ENGINEERS RULE OF THUMB – Turn the Castor Wheel (or equipment) Upside Down. Now, using as much force as possible (by hand) spin the Castor Wheel about its swivel bearing. You should expect the swivel mechanism to rotate no more than 1 to 1.5 times. Anymore, and it signifies little friction in the Swivel Race to control flutter.
You can view a demonstration video below (including Slow Motion):
Misalignment of the Castors to Ground – If the Castors are not installed correctly this can cause unequal forces to be applied on each of the Castors. For example, if the Castors are not aligned in the vertical position this can cause one of the wheels to be very lightly loaded or even come off the ground. This significantly reduced the force, and hence the ground friction for that particular castor wheel. Less Friction means that the resistance to flutter is reduced. This is typically the main cause of Castor Wheel Flutter on Supermarket Trolleys.
Use a Longer “Trailing Distance” – The distance between the Centre of the Castor Swivel and Wheel Hub Centre is known as the “Trailing Distance”. Increasing the “Trailing Distance” will increase the natural frequency of the wheel. The Higher the Natural Frequency, the Faster the Wheel must spin in order to flutter. One of the easiest ways to increase the Trailing Distance is to increase the diameter of the Castor Wheel!
Wheel Friction – The Friction of the Castor Wheels Tread should be considered. Increased friction not only increased the stability and control of the equipment under normal operation, but also dampens the forces generated by Castor Wheel Flutter. Rubber has a much higher coefficient of friction compared to Nylon.
Bulldog Castors™ – Here to Help
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 firstname.lastname@example.org or +44 (0)116 2970521