Solution Summary: User Vibration Protection
User Vibration Protection (UVP) technology could reduce the operating vibration of power hand tools significantly. Using the ideas of prevention through design, while the tool is operating, a built-in counterweight mechanism moves in the opposite direction of the tool, resulting in vibration absorption. In addition to the counterweighting technology, shock absorbing handles also could reduce the vibration before it reaches workers’ hands. This solution could be considered as an engineering control as it provides a barrier between the source of hazard and the worker.
User Vibration Protection (UVP) is an engineering control that can reduce the risk of Hand-Arm Vibration Syndrome (HAVS) for workers who operate power hand tools. Some available models produce 33% lower vibration compared to conventional models (Figure 1). Using innovative counterweighting systems along with vibration-absorbing handles could enhance the safety features of a wide range of construction tools, such as rotary hammer drills, sabre saws, grinders, and demolition hammers.
Figure 1. Power hand tools equipped with UVP system; left: grinder, middle: rotary hammer, right: demolition hammer. (Source: http://www.hitachipowertools.com/us/)
Hitachi is the only brand in the U.S. that incorporates UVP technology in a wide range of its handheld tools. UVP equipped models of grinders, demolition hammers and rotary hammers are available on the market. 15 ampere 7" and 9" angel grinders (Figure 1, left) can reduce the tri-axial vibration up to 30% to only 3.3 meters per second squared which is the lowest vibration in their class. Soft elastomer grips also help minimizing the vibration that could reach to a worker's body. Using an internal system of counterweights and springs along with shock-absorbing mechanism and soft grips in hammers (both rotary and demolition) greatly reduces vibration for comfortable extended use and fewer vibration related injuries.
Working with construction powered hand tools such as drills, saws, etc. can pose several risk factors to the operators, including excessive vibration. Gemne (1997) has listed some of the disorders that vibration could cause including: Hand-Arm Vibration Syndrome (HAVS), disturbance of finger skin circulation, diffusely distributed finger neuropathy, carpel tunnel syndrome etc. Excessive amount of vibration could affect different parts of body:
- Vibration could affect the nerves of the fingers and result in initial numbness and tingling. While these injuries are recoverable if cured in time, neglecting early symptoms could lead to irreversible damage, such as impaired sensibility and limited control even on simple tasks. Carpal tunnel syndrome (caused by pressure on the median nerve in the wrist) also could coexist with HAVS.
- Blood vessels also can be damaged by vibration. “Vibration induced White Fingers” (VWF) or “secondary Raynaud’s disease” happens when the blood vessels become narrower due to vibration and cannot receive oxygen from the blood, causing white spots on operators’ hands and fingers. This results in major discomfort and pain when the blood flow returns to normal. Symptoms include numbness, tingling, ashen skin, loss of feeling and control. In more serious cases, in which the whole body is affected by vibration, workers experience low back pain and numbness in the upper legs.
- Muscles also are affected by vibration. While it is usually difficult to show the damage of high-vibration tools to the muscles, “microscope studies have shown that there may be significant changes in the fibers” (https://www.vibrosense.eu/knowledge-bank/medical-background/hand-arm-vibration-injuries).
OSHA classifies all of these injuries under “Musculoskeletal disorders" (MSDs), and warns that “excessive vibration, usually from vibrating tools, can decrease blood flow, damage nerves, and contribute to muscle fatigue”. (https://www.osha.gov/Publications/osha3125.pdf). Obtaining low-vibration tools is a practical way to minimize the impacts of vibration on workers and to avoid subsequent injuries.
How Risks are Reduced:
As mentioned above, the UVP system acts mainly through two separate mechanisms: 1) a spring-type dynamic vibration absorber and 2) a vibration-absorbing handle. The main mechanism consists of a counterweighting dynamic vibration absorber as demonstrated in Figure 2. In vibration cycles, every time the force is directed toward the worker’s body, the vibration absorber causes the weight to move in the opposite direction and absorb the force and reduce the movement. In the opposite situation, when the force tries to move the tool away from worker’s body, the springs move the weight toward the body, which cancels the effect of tool’s force (Figure 2a). In some cases, coil springs assist with weight movement (Figure 2a), while in others leaf springs have been used to create the same effect (Figure 2b).
Figure 2. UVP system in Rotary hammer drills. a) with coil spring, b) with leaf spring
The second simpler mechanism places several dampers (usually rubber) around the handle’s shaft to absorb the vibration and reduce its transfer to the operator’s body. Figure 3 depicts this mechanism in a demolition hammer.
Figure 3. Vibration absorbing handle for demolition hammer
Considering the broad range of occupations that are affected by hand-transmitted vibration hazards and the annual cost of HAVS related injuries to the economy (a study in UK in 2002 has estimated this cost to be more than $575 million), several studies have been investigated the main risks caused by vibration, the measures to quantify those risks, and innovative solutions that can reduce the exposure amount/time for workers. For instance, Gemne (1997) explains the injuries and the methods to assess their symptoms. In another study, Gemne and Saraste (1987) analyzed the effect of vibration on bones and joints and concluded that there is an association between work with low-frequency tools such as chipping hammers and the prevalence of premature elbow and wrist osteoarthrosis. Govindaraju et al. (2008) tested the impact of several ranges of vibration frequency on rats' tails and showed that the severity of nerve damage did not correlated with specific frequencies. So, they concluded that both high and low-frequencies could be dangerous to a worker's body.
Some studies have focused on methods of measuring the amount of vibration as the main challaenge to compare different tools and the amount of risk they expose to the workers. Dong et al. (2004, and 2008) for example, attempted to quantify the amount of energy that is absorbed by different parts of a worker's hand and arm. They found that with low-frequency tools (e.g., rammers) the vibration power was mainly distributed in arms and shoulders, while with high-frequency tools (e.g., grinders) the power would concentrate in fingers and hand.
Finally, several studies have examined the applicability of vibration protection ideas and technologies to powered handheld tools. Pinto et al. (2001) have tested the performance of five commercially available anti-vibration gloves to assess the isolation effectiveness of the gloves and develop test methods to estimate the protection of them. Golysheva et al. (2003) introduced a passive system to reduce the amount of vibration transited to operator's body. The proposed system consisted of two technologies: 1) a vibration isolator was placed between the handle and casing of machine and 2) dynamic absorbers were attached to the handle to reduce machines acceleration. However, the effect of this system was not quantified in the study. In another study, Hao and Ripin (2013) proposed an imposing node technique to tune the frequencies of two Tuned Vibration Absorbers (TVAs) that were attached to the tool to reduce the amount of vibration for grass trimmers. Their field test results confirmed that the TVAs could reduce the total value of vibration by 58% during the cutting operation.
Effects on Productivity:
One of the main impacts of vibration on the human body is fatigue, which forces the tool’s operators to take several breaks from work and reduce their productivity. Using low-vibration tools will directly decrease fatigue and enhance productivity of workers. Using safer tools also helps workers mentally, leading to higher productivity.
Behzad Esmaeili, Ph.D. - University of Nebraska- Lincoln
Pouya Gholizadeh - University of Nebraska- Lincoln
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