Project 1
Introduction
Forearm crutches are used by many when recovering from an injury or by those who have disabilities. They serve as a means of stability and balance by transferring the weight of one’s lower body to the upper body. One or two crutches are used depending on the severity of the injury or disability.
The use of crutches dates back to Ancient Egypt, where the Egyptians would use tree branches to help balance themselves. Emile Schlick patented a walking stick in 1917 with a support for the upper arm of the user and A.R Lofstrand Jr. patented the first walking stick that could be adjusted to the user’s height. In 1988 Thomas Fetterman patented forearm crutches because of the dangers of slipping he experienced with a normal crutch. He would later design a crutch tip with a shock absorbing gel cap to make forearm crutches more comfortable for those who used them.
Parts of Crutch
Strength, comfort, coordination and comfort are the four major criteria that a pair of crutches needs to satisfy. Aluminium alloys, both strong and light, make up the majority of a crutch providing the stability to support the user’s weight without bending or deforming the metal. The cuffs of forearm crutches are made from plastic and are designed to be able to bend so that the user can place their arm through the cuff while still being stable when the user is walking. The handle of the crutches can be plastic or rubber depending on the manufacturer or customer needs. Rubber is more comfortable while plastic is more durable.
The crutch tip is usually made from rubber encasing the shock absorbing gel. The height adjusters, bolts and screws are usually made from stainless steel. The forearm crutch consists of a frame, grip handle, cuff, tip and height adjusters. Each of these parts is made from materials with mechanical properties that best fit the requirements of each individual part. The frame of the forearm crutch is usually made from an aluminium alloy. Aluminium has a low density and thus a low weight, to make it mobile and easily manoeuvrable. It also has high strength so that the crutch will support the full weight of a person without bending or buckling. Aluminium is also easy to machine and corrosion resistant to ensure a long life.
Plastic cuffs are used on the forearm crutch. It is usually made from a plastic – such as Nylon-6/6. It has good abrasion and hydrocarbon resistance, to handle the repeated friction and contact to skin when walking. It also has high strength and stiffness to provide sufficient support to the user. The grip handles of the forearm crutch can be made from either the same plastic as the cuffs or from rubber. The plastic provides more wear resistance and durability, but the rubber is more comfortable and can also absorb more shock.
The tip of the forearm crutch is made from rubber. The rubber provides plenty of grip and stability to prevent slipping when walking. It also has good shock absorption to dampen the shock of walking. There are many types of height adjustment mechanisms, including adjustment pins and clamps. These mechanisms are usually made of stainless steel. This provides high strength and little deformation to provide a stable link between the various parts of the main frame. It also has a high abrasion resistance to make it durable and usable outside.
Manufacturing Process
The manufacturing process of making forearm crutches can be divided into two main operations: The making of the aluminium hollow tubes and of the plastic arm cuffs, hand grips and the replaceable rubber tips. Although a few crutch manufacturers prefer buying readymade hollow tubes which are available in different sizes and shapes, many other prefer manufacturing them themselves to achieve the precision and quality they desire.
The aluminium hollow tube is made through a process called drawing, through which a highly heated starting billet (stock) is drawn through a die to reduce its diameter and stretch its length by applying high pressure. This requires the starting billet to be hollow initially and the process improves the dimension accuracy of the desired tube and its surface finish. The initial heating process allows for the material to be soft but still retain its solid property, consequently keeping the internal properties intact. Once this drawing process is finished, the tube is rapidly cooled through water quenching. Depending on the desired shape of the crutch, the hollow tube is then bent and holes punched through at various points along its length. This is for the final assembly when the other parts need to be attached.
The arm cuffs, the hand grips as well as the rubber tips are manufactured through an injection moulding process, making use of an IMM (injection moulding machine) or injection press. This machine can be mounted with different types of mould cavities for the different shapes desired of the parts to be made. The process starts with feeding the plastic polymer into the heating unit of the machine to melt it, the molten material is then pressed through high pressure injection into the specific mould cavities to form either the arm cuff, the hand grip or rubber tip, the material goes through an opening called sprue. The cavities are then filled and more material is poured into the machine than just to fill the moulds, this is to compensate for the shrinkage that will occur once the material starts to solidify. The moulds are then held together until the part is fully shaped and cooled, thereafter the moulds are opened and the part is ejected from the machine. This specific manufacturing is best suited for making plastic materials such as the arm cuffs and hand grips as well as rubber because of its precision and accuracy of shaping parts and also for the complexity of shapes of the parts it can make.
Once all the parts are finalised and checked for quality inspection, they are then assembled to form the actual forearm crutch as a finished product.
Conclusion
By curving the crutches, they in essence become shock absorbers which will reduce the impact on the user’s upper body immensely. It will also allow for energy retention to propel the user forward ounce the crutch straightens which will save the user a lot of effort and make traveling longer distances with crutches less painful and tiring. The fact that the crutch is now only one piece means that there are fewer parts, which simplifies manufacturing and will extend the life of the crutch as there are less parts which can break. Graphite is also more durable than aluminum and is able to bend without yielding, which is a big advantage for the energy retention. The graphite is also lighter than the aluminum which will save the user energy. Aluminum is an easily recycled material, therefore it seems illogical to replace the aluminum in the design with graphite, which is not easily recycled, but sustainability is not only about recycling. Graphite is a very durable material which will not be greatly affected by wear and tear. Therefore the crutches can be reused multiple times without any drop in performance or appearance.
References
https://group10crutches.wordpress.com/
https://www.sciencedirect.com/science/article/abs/pii/S0957415821001483
https://www.mdpi.com/2076-3417/9/23/5032
https://pubmed.ncbi.nlm.nih.gov/9821913/