MKsafety® popularizes the knowledge of safety shoes. The difference between the safety sole and the shoe upper is that it has to carry the weight of a person’s whole body, and it may even be under pressure of about 2 to 4 times the body weight during rapid movement in operation. Not only that, but it is also required to have the functions of abrasion resistance, shock absorption, elasticity, slip resistance, sprain resistance, comfort and air permeability, suitable softness and hardness, light weight and resistance to flexure. So how can we meet so many requirements? This must require safety shoe manufacturers to conduct force analysis from the positioning of sports shoes, and provide a basis for shoe sole design through detection and analysis methods.
Now we analyze the influence of several main properties of safety shoes on the force of the sole for reference.
The folding resistance of safety shoes is a priority factor for labor workers, and it is also a physical property index that the domestic quality supervision department pays great attention to. The test method is based on the national standard of GB21148-2007, and the sole of the metatarsophalangeal joint is cut 5mm in length. The test is performed at a bending frequency of about 4 times per second, which is extremely demanding. Therefore, when the sole is designed and produced, the thickness of the sole bending groove and the toughness of the substrate should be considered. Generally, the design of the bending groove is based on the arc shape, which can avoid stress concentration and disperse the bending force and extension force. At the same time, the back of the bending groove should be thickened to more than 2.0mm by means of stiffeners.
2. Abrasion resistance
At the current stage when domestic consumption is still at a relatively low level, the durability of safety shoes is undoubtedly the most concerned by consumers. In addition to the folding resistance of the sole, wear resistance is also one of the determinants of durability.
2.1 Peripheral wear
In general, 10-15mm inward from the edge of the sole periphery (forefoot, mid-waist, heel) is the most worn part, but the degree of wear is less than the toe and heel. The reason is that the thumb, little finger and heel are the main stress points.
2.2 Toe toe wear
The toe toe is less worn than the heel, and the toe toe’s outer side is less worn than the toe’s inner side. The reason is that when walking or running, the heel touches the ground first, and when the foot is lifted off the ground, the thumb finally lifts and pushes forward and generates friction.
2.3 heel wear
The heel is the main part of abrasion. According to statistics, more than 80% of people’s feet land on the outside of the heel first, and because the feet deviate toward the inside when starting and landing, the outside wear is greater than the inside.
2.4 Wear-resistant design
According to the above analysis of abrasion, some safety shoes with high abrasion function requirements must be made of rubber with good abrasion resistance as the outsole, such as the Italian super abrasion-resistant vibra rubber sole. Or specially designed wear-resistant rubber patches on the inside of the toe cap and the outside of the heel.
When exercising in the workplace, there are often sudden stops and sudden turns. At this time, insulated shoes are required to have high anti-slip properties. Otherwise, it will not only cause work-related accidents, but also may cause greater unnecessary safety hazards. However, under the condition that other aspects of the sole remain unchanged, the abrasion resistance and anti-slip performance are inversely proportional. The better the abrasion resistance, the lower the anti-slip performance; the better the anti-slip, the better the anti-slip performance. The grindability is poor. Therefore, a large number of tests and comparisons should be conducted to find the best combination of wear resistance and anti-slip properties for design and production. Of course, it is necessary to consider the design of the sole texture from the function of the insulated shoes, so as to determine the best balance between wear resistance and anti-slip.
Support is a more complicated issue. Specifically, the shoe is designed to be tight and thick to ensure the strong support of the foot to the human body. This compact design can overcome the dispersing force of the foot to ensure upward support. Generally, a hard support piece (such as a TPU piece) is used to extend from the waist to the heel of the shoe and insert it into the midsole or between the outsole and the midsole. It can stably support and fix the foot, and can also prevent sprains during exercise.
The breathability is not limited to the upper, but the sole also requires breathability. Because when wearing shoes normally, the temperature of the human foot skin can reach 34-35℃; and during intense exercise, its temperature will reach 43-49℃; in addition, the rougher and more complicated the design of the large bottom pattern, it will rub against the ground. The greater the force, the higher the heat. Therefore, the design of the breathability of the sole is very necessary, but at the same time its waterproofness must be considered. Generally, a layer of breathable waterproof glue is applied to the transom.
6. Shock absorption
A person walks on foot under a load, about 600 to 700 steps per kilometer. This means that for every kilometer of walking, one foot has to withstand 600 to 700 gravity impacts. If it is intense exercise, the impact will be even greater. Someone has done statistics. When a person is running, the moment his foot touches the ground, the impact force on the ground will reach 2 to 4 times the weight of the human body. If the shoe does not have a good shock absorption system to relieve this shock, it will definitely make your feet feel tired, and it will also cause an impact on the brain. In general, a midsole with a certain degree of elasticity (such as EVA, PU) or an elastic pad (such as EVA, PU) is embedded in the heel of the outsole to reduce impact. At the same time, the effect of energy return can be achieved (energy return means that after the sole impacts the ground, the kinetic energy is absorbed by the elastic body deformed under pressure. Later, before it leaves the ground, the shape of the elastic body recovers. Return the energy to the wearer so that the wearer can run faster and jump higher).
7. Sprain resistance
The human foot is composed of 26 bones. The human walking process is a very complex and scientific process of coordinated movement of bones and muscles. From touching the ground to lifting off the ground, the foot receives an upward impulse and forward friction. When the foot touches the ground, it receives a very strong force. A large momentum requires a certain force to be exerted on the ground to obtain forward friction before lifting it off the ground. In this process, the analysis of the movement process of the human foot is like this: More than 80% of people’s feet land on the outside of the heel first. At this time, the heel axis is slightly deviated outwards. When landing, the foot receives a great impulse. Flip inward naturally to disperse the damage to the foot joints caused by ground impact. Finally, the heel axis changes from an outwardly deviated position to a vertical ground position; before being lifted off the ground, the relevant muscle groups and joints contract and tighten to provide The force on the ground. At this time, the heel axis changes from the vertical ground position to the outward oblique position to provide the force. In this process, there are usually two situations: overturning and insufficient turnover. If the foot is turned inward after landing, the heel axis crosses the vertical plane and inclines inward, then when the foot is off the ground, it is too late to adjust to the outward incline position, so that the musculoskeletal cannot be fully prepared, and it is easy to cause sports sprains. In order to overcome the foot Flat feet that collapse under the arch are prone to sports sprains. The soles are generally designed as dual-density elastomers. A relatively high-density material is designed on the inner side of the heel to resist the sprains caused by overturning of the foot. Another situation is the phenomenon of insufficient turnover that often occurs in people with high arches. Because the joints of the feet of people with high arches are generally stiff, when the feet are on the ground, they often cannot completely flip to the vertical position, which is not enough to resolve the impact of the ground on the feet and various joints, which may easily cause sprains. People with this type of foot, The outer side of the shoe is often worn out, and the shoe last that has good shock absorbers and fits the foot shape is the best choice. Of course, it can also be combined with the design of the insole to enhance torsion resistance and reduce fatigue.
Lightweight is one of the factors that directly affect the comfort of labor insurance shoes, and it is also a key factor for laborers to perform optimally. Because the wearer consumes a lot of physical energy during exercise, it is required to minimize the load outside the body weight. Safety is the main consideration. On the basis of ensuring the function of safety shoes, the weight of the shoes should be reduced as much as possible, so that the laborers can walk lightly. Foreign countries have long imposed upper limits on the weight of safety shoes, and PU.EVA has been widely adopted in China in recent years. Foamed elastomers such as MD and injection EVA are used as the midsole.