1.0Bulldog Castors Bloghttps://www.bulldogcastors.co.uk/blogrich600338<blockquote class="wp-embedded-content" data-secret="ThNW8hFPYR"><a href="https://www.bulldogcastors.co.uk/blog/castor-wheel-compression-set/">Rubber Castor Wheel Compression Set</a></blockquote><iframe sandbox="allow-scripts" security="restricted" src="https://www.bulldogcastors.co.uk/blog/castor-wheel-compression-set/embed/#?secret=ThNW8hFPYR" width="600" height="338" title="“Rubber Castor Wheel Compression Set” — Bulldog Castors Blog" data-secret="ThNW8hFPYR" frameborder="0" marginwidth="0" marginheight="0" scrolling="no" class="wp-embedded-content"></iframe><script type="text/javascript"> /*! This file is auto-generated */ !function(d,l){"use strict";var e=!1,o=!1;if(l.querySelector)if(d.addEventListener)e=!0;if(d.wp=d.wp||{},!d.wp.receiveEmbedMessage)if(d.wp.receiveEmbedMessage=function(e){var t=e.data;if(t)if(t.secret||t.message||t.value)if(!/[^a-zA-Z0-9]/.test(t.secret)){for(var r,a,i,s=l.querySelectorAll('iframe[data-secret="'+t.secret+'"]'),n=l.querySelectorAll('blockquote[data-secret="'+t.secret+'"]'),o=new RegExp("^https?:$","i"),c=0;c<n.length;c++)n[c].style.display="none";for(c=0;c<s.length;c++)if(r=s[c],e.source===r.contentWindow){if(r.removeAttribute("style"),"height"===t.message){if(1e3<(i=parseInt(t.value,10)))i=1e3;else if(~~i<200)i=200;r.height=i}if("link"===t.message)if(a=l.createElement("a"),i=l.createElement("a"),a.href=r.getAttribute("src"),i.href=t.value,o.test(i.protocol))if(i.host===a.host)if(l.activeElement===r)d.top.location.href=t.value}}},e)d.addEventListener("message",d.wp.receiveEmbedMessage,!1),l.addEventListener("DOMContentLoaded",t,!1),d.addEventListener("load",t,!1);function t(){if(!o){o=!0;for(var e,t,r,a=-1!==navigator.appVersion.indexOf("MSIE 10"),i=!!navigator.userAgent.match(/Trident.*rv:11\./),s=l.querySelectorAll("iframe.wp-embedded-content"),n=0;n<s.length;n++){if(!(r=(t=s[n]).getAttribute("data-secret")))r=Math.random().toString(36).substr(2,10),t.src+="#?secret="+r,t.setAttribute("data-secret",r);if(a||i)(e=t.cloneNode(!0)).removeAttribute("security"),t.parentNode.replaceChild(e,t);t.contentWindow.postMessage({message:"ready",secret:r},"*")}}}}(window,document); </script> https://www.bulldogcastors.co.uk/blog/wp-content/uploads/Castor-Wheel-Compression-Set-Test.jpg16321224Rubber Castor Wheel Compression Set (A.K.A Permanent Set) Within this article we will be looking at the “Cause” & “Effect” of Castor Wheel Compression Set……   Rubber as a Castor Wheel Material- A Brief Introduction  The use of Rubber as a material for Castor Wheels has been in existence for more than a century. Produced from the naturally occurring Latex of the Para Rubber Tree, Natural Rubber has become a common material within the Castor Wheel Industry. In more recent times, Synthetic, Manmade Rubbers have been developed to simulate the properties of Natural Rubber with the addition of other specific mechanical properties, such as Oil Resistance, which Natural Rubber does not offer. However, scientists have been unable to fully emulate the full extent of the properties of Natural Rubber, and therefore Natural Rubber is still the material of choice for many applications. Permanent Set is a characteristic of most rubber materials. It is usually seen as an unfavourable characteristic, particularly within the Castor Wheel Industry, that should be controlled and kept to a minimum.   What is Castor Wheel Compression Set? Compression Set of a material is defined as “The Permanent Deformation of a Material after a Force has been Removed”. It should also be noted that the opposite is possible, called Tensile Set, where the material has a Tensile Force applied and any Permanent Deformation is considered as Tensile Set. Lastly, it should also be noted than both Compression Set and Permanent Set can be referred to, interchangeably, with the term “Permanent Set”. Castor Wheel Compression Set is a undesirable characteristic whereby a wheel develops flat spots (permanent change in shape). Example of Compression Set on Rubber Castor Wheels Imagine a Trolley, mounted with four Rubber Castor Wheels. The Trolley is carrying 100Kg of Goods/Equipment.  This 100Kg applies a force to the Rubber Wheels, which in turn deforms the Rubber Wheels, let’s say by 6mm. When the Goods/Equipment are removed from the Trolley the deformation on the Rubber Castor Wheels will recover.  Any “Non-Recovery” is considered permanent Set. So, using the above example, when the 100Kg Weight is removed, the wheels recover, let’s say by 5mm, leaving a 1mm Permanent Set. What Causes Compression Set on Rubber Castor Wheels The Causes of Castor Wheel Compression Set are mostly due to the formulation of the rubber compound. If the end user is using a reputable Castor Wheel manufacturer, the Control on Compression Set will have been taken into account when the rubber compound was formulated, and therefore the end user should focus on how Compression Set will “Effect” performance, rather than the “Cause”. For those of you out there which wish to understand the causes Compression Set, we firstly need to look into the basic chemistry of rubber…… Rubber, whether Natural or Synthetic, comprises a molecular structure of Long Chain, Cross Linked Polymer Molecules.  Simply put, the molecules of the rubber, which are all linked together, are randomly arranged and randomly linked.  They are not aligned or linked in a uniform manner. When Stress and Strain is applied to the Rubber material these randomly arranged molecules will extend, compress and change position.   Each time Stress and Strain is applied to the material the arrangement of molecules will change. These randomly aligned and linked molecules are the basic feature of a “Visco-Elastic” material – a material that can both store and dissipate energy. This Visco-Elastic property is relatively unique amongst Polymers, where the Viscous property of the material absorbs and dissipates energy (ideal for shock absorption), whereas the Elasticity of the material stores and rebounds the Energy (ideal for wheels rolling over rough ground). Below you can see a “Load vs. Compression” graph for a Visco-Elastic Material. As can be seen on the graph, the deformation of the Visco-Elastic Material follows a different path on the “Load” and “UnLoad” cycle.  The Unload cycle shows that the materials deformation is slower to recover compared to the Load Cycle.  This difference in recovery is known as Hysteresis, which is  due to Energy Dissipation by generation of heat. The level of hysteresis is defined as a percentage, and is calculated by measuring the area within the loop (shown as the Red shaded area), compared to the total area (shown as the Red + Blue shaded area) This Hysteresis is an important factor when considering Castor Wheel Compression Set.  –  Further details on Hysteresis can be found HERE. Now, back to Compression Set…….. The Factors Affecting Compression Set on Rubber Castor Wheels: HYSTERESIS – The Formulation and Rubber Type will affect the ratio between the “Visco” &. “Elastic” properties of the material.   A Natural Rubber Compound may have a Low Level Hysteresis (Visco) Level of between 5-15%, whereas a Butyl rubber may have a Hysteresis (Visco) Level between 25-35%. A high level of Hysteresis, and hence its inability to recover at the same speed, results in energy being converted into heat and ultimately the potential for the rubber not to fully recover. It should be noted however that for Testing & Quality Control purposes, whilst Hysteresis measurement is a good indication of the possibility of Permanent set, it is possible that some rubbers will fully recover after a short period of time.    It can therefore be considered that Hysteresis is a “Indicator” for potential Castor Wheel Compression Set. FILLERS – Raw Polymer is Highly resilient, however in its raw form many of the required mechanical properties, such as Tensile Strength and Abrasion Resistance are poor.  Therefore, the Raw Polymer is mixed with other materials (known as Fillers).  The types and percentage of Fillers have a major impact of Compression Set on Rubber Castors. The Most Common Filler is Carbon Black.  It is known as a reinforcing filler and provides excellent Tensile Strength and Wear Resistance and many other beneficial properties required for Castor Wheels. However, other fillers are available (such as Calcium Carbonate or Clay) which are normally used to reduce the cost of the Rubber Compound and can increase the Hysteresis percentage....