{"id":2712,"date":"2016-05-23T19:40:07","date_gmt":"2016-05-23T19:40:07","guid":{"rendered":"https:\/\/www.bulldogcastors.co.uk\/blog\/?p=2712"},"modified":"2018-11-02T04:10:44","modified_gmt":"2018-11-02T04:10:44","slug":"castor-wheel-flutter","status":"publish","type":"post","link":"https:\/\/www.bulldogcastors.co.uk\/blog\/castor-wheel-flutter\/","title":{"rendered":"Castor Wheel Flutter"},"content":{"rendered":"

What is Castor Wheel Flutter?<\/strong><\/h2>\n

Castor Wheel Flutter<\/strong> 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.<\/p>\n

Many people will have experienced Castor Flutter at the supermarket.\u00a0 Ever noticed how one of the wheels of the Shopping Trolley seems to be rotating of its own accords?\u00a0 Yes!, well that\u2019s called Castor Wheel Flutter.<\/p>\n

Castor Wheel Flutter is caused by harmonic excitation of the castor wheel.\u00a0 Simply put, when the wheel tread deforms and rebounds, due to force and movement, oscillations are caused.\u00a0 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.<\/p>\n

In order to understand how these \u201cOscillations\u201d cause flutter, we first need to understand each stage of the wheels oscillation. It\u2019s a relatively simple 2 stage process, as follows:<\/p>\n

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  1. Firstly, the Rubber Wheel<\/a> deforms due to the weight and force that is applied to the equipment. For arguments sake, let\u2019s say the wheel takes 50 milliseconds to maximum deformation.<\/li>\n
  2. Secondly, the Rubber Wheel recovers (rebounds) from the deformation. Again, for argument sake this takes 50 milliseconds to recover.<\/li>\n<\/ol>\n

    These are the 2 stages that occur as the castor wheel is travelling.\u00a0 Each Time Point 1 and Point 2 occurs, it is considered 1 oscillation.\u00a0 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.<\/p>\n

    So, how does this cause Flutter?<\/strong><\/h3>\n

    Castor Flutter is due to the \u201ctime phase lag\u201d between the \u201cinitial deformation force\u201d and the \u201creactive rebound force\u201d.\u00a0 In the above example these two forces lag each other by 100 milliseconds (0.2 Seconds).\u00a0 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.\u00a0 The next oscillation then over corrects in the other direction, causing the wheel to swivel from side to side.\u00a0 This action repeats itself over and over.<\/p>\n

    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).\u00a0 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)<\/p>\n

    Considering the above the 2 primary factors that cause Castor Wheel Flutter are \u201cSpeed of the Wheel\u201d and \u201cWeight upon the Wheel\u201d.<\/p>\n

     <\/p>\n

    Why do you need to consider Castor Wheel Flutter?<\/strong><\/h3>\n

    For small appliances, or equipment travelling at slow speed Castor Flutter is not an issue.\u00a0 However there are many applications where Castor Flutter can have significant consequences.<\/p>\n

    To give an example, consider a warehouse trolley in a busy distribution centre.\u00a0 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.\u00a0 If one of the Castor Wheels was to flutter, the operator may lose control of the trolley.\u00a0 This could result in colliding with an object, running over a person\u2019s foot, or even tipping of the trolley.\u00a0 This could have a serious impact on both the commercial and liability responsibilities of the warehouse owner.<\/p>\n

    Castor Flutter can cause Loss of Control, Reduced Handling, Wear of Bearings and Wheel Tread, and Excessive Noise.<\/p>\n

    It should be noted that different applications may require different levels of control.\u00a0 From Shopping Trolleys to Aerospace and Luggage Handling Equipment, the consequences of castor flutter are different.\u00a0\u00a0 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.<\/p>\n

     <\/p>\n

    How to control a Castor Wheels Flutter<\/u><\/strong><\/h3>\n

    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).\u00a0 However from a practical, commercial & efficiency perspective this may be difficult to achieve.<\/p>\n

    It is therefore normal practice to \u201cControl\u201d the Castor Flutter, rather than \u201cEliminate\u201d it, and it should be expected that all Castor Wheels will \u201cflutter\u201d at a given speed and force.<\/p>\n

    Below, we will discuss some of the ways in which Castor Wheel Flutter can be controlled:<\/p>\n

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    • Swivel Ball Race Tolerance & Fit<\/strong> – \u00a0If 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.\u00a0 Less Friction = Less Resistance to Swivel.\u00a0 If the Ball Race Swivel has a good Fit, the over-correction forces caused by the oscillations will be dampened by friction.\u00a0 If the Castor Wheel has been manufactured to a low standard, or where the castor bearings have worn, resulting in a \u201csloppy\u201d fit, this will increase the likelihood of Castor Wheel Flutter.<\/li>\n
    • Lubricating Grease<\/strong> – The Viscosity of the Grease on the Swivel Bearings affects the dampening effect of the forces.\u00a0 If the Grease is of High Viscosity it will dampen the forces quicker, resulting in energy loss, and reduced correction forces.<\/li>\n
    • ENGINEERS RULE OF THUMB<\/strong> –\u00a0 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.\u00a0 You should expect the swivel mechanism to rotate no more than 1 to 1.5 times.\u00a0 Anymore, and it signifies little friction in the Swivel Race to control flutter.<\/li>\n<\/ul>\n

      You can view a demonstration video below (including Slow Motion):
      \n