ERGONOMIC QUALITY AS A MEANS OF ...

21 downloads 0 Views 380KB Size Report
pneumatic hammers. Due to HAVS, employees who use hand tools that create vibrations are at risk for developing, among others, numbness in the fingers.
Marcin BUTLEWSKI, Ph.D., Engr. Department of Management Engineering Poznan University of Technology

ERGONOMIC QUALITY AS A MEANS OF PROVIDING POWERED HAND TOOLS’ SAFETY Summary. The following article presents the main features that determine the ergonomic quality of powered hand tools, as well as the premises of design allowing for the creation of this quality as a safety determinant of powered hand tools. Presented are the results of a survey of tool users, designed to determine the characteristics of tools that increase their safety and ergonomic quality. Keywords: tool safety, powered hand tools, ergonomic quality of tools

Introduction Powered hand tools can be, and are in many cases, the cause of accidents in the workplace. In 2004, 3.5% of all serious accidents during work were the result of contact between the operator’s body and sharp hand tools in motion [25]. Powered hand tools can cause permanent injury to their users as well as occupational diseases. In the years 1984-1994, the percentage of occupational diseases resulting from a large number of repetitive movements during work (carried out using hand tools), increased from 18% to 69% [1]. In Poland, the number of musculoskeletal disorders caused by hand tools has consistently decreased since 1990, when it was about 300 cases, 172 in 2000, and 104 in 2003 [31]. However, the decrease in the number of occupational diseases caused by working with hand tools does not necessarily mean that there is an increase in the ergonomic quality of these tools, as this trend may not include otherwise classified injuries and occupational diseases. In the case of the use of hand tools by workers aged 17 to 55 years, up to 20% showed symptoms of cumulative disorders of the musculoskeletal system [2]. In the years 1982 – 1986, 12% of all accidents registered in the United States were caused by hand tools, with 20% of them arising as a result of the use of powered hand tools that are of too poor quality [24]. In turn, MSD’s - musculoskeletal disorders, and in the context of work, Workplace Musculoskeletal Disorders (WMSDs) [16] are the most commonly reported ailment resulting from the way of performing work in the United Kingdom [7]. Hence, it seems important to recognize the features responsible for the ergonomic quality of hand tools, thereby reducing their negative impact on the health of users. This approach will allow a better formation of safety in the workplace using powered hand tools, which will not only be associated with the prevention of acute injuries. Impact of powered hand tools on the safety of users In addition to the possibility of a sudden injury to the user, hand tools that are powered (electrically, pneumatically, or magnetically) are a source of a number of

afflictions – musculoskeletal disorders. Powered hand tools that create vibrations can cause, among other things, “Hand-arm vibration syndrome” (HAVS). This syndrome was first identified in the United States in 1918 in grinders, who used pneumatic hammers. Due to HAVS, employees who use hand tools that create vibrations are at risk for developing, among others, numbness in the fingers (paresthesia) and symptoms of "white fingers" - pale fingers - ischemia [19]. The occurrence of excessive emission of vibrations by powered hand tools is still a major problem for today's employees; in order to reduce their impact on the body of the operators active anti-vibration gloves are used [14]. The movements performed during labor using powered hand tools are often characterized by repetition or unfavorable continuity. A repeated maximal extension of the wrist (similar to when throwing a ball) is the most important factor causing damage to the tendon at the elbow, known as the lateral epicondyle inflammation or "tennis elbow". The cause of this ailment can also be a continuous grasping without stabilization of the wrist, for example, when holding a paint spray gun [5]. It is similarly disadvantageous, in the case of automated hand tools, to keep the finger in a curved position and with a high external pressure (typical for the compression of trigger devices). However, an especially frequent accident during use of these devices is the encounter of a blade and/or the moving parts of the machinery with the body of the operator. Injuries caused by these tools are cuts and bruises, eye damage done by spalls from treated surfaces, and electric shock [22]. Mechanical power transmission, in particular, a high torque generated by hand tools, may cause unfavorable extension of the forearm muscles. Such a situation can especially escalate during a sudden increase in torque, for example, while working with an electric drill [26].

Fig. 1. Muscle extension with an increase in torque [26 pg.2]

Studies carried out throughout the world have shown that hand tools are a major cause of the emergence of occupational diseases associated with injuries arising from the accumulation of micro-traumas, known as CTD (Cumulative Trauma Disorders) [11]. The main symptoms of CTD are ailments of the hands and joints

2

which lead to an inflammation of the tendons, block the flow of blood, and strain muscles [27 pg.16]. Overload syndromes is a general name for a group of musculoskeletal disorders caused by repetitive actions, a forced position, and the squeezing or stretching of anatomical structures. They are the effect of mechanical loads in excess of physical strength and endurance of the static-dynamic components and may affect all structures of the musculoskeletal system [8 pg.5]. Typically, a worker exposed to overload (micro-trauma) does not identify the relationship between the symptoms and the operations performed. The cause of overload syndromes is performing simple actions: twisting, reaching, squeezing, pulling, and pushing. These actions in themselves are not dangerous, but their repetition determines the possible emergence of persistent injuries. Named are the following main groups of labor causing overload syndromes: [13 pg.12]. − rapid and repetitive movements of low intensity but high frequency − static muscle loads, arising due to an improper position of the body or limbs or a badly organized workplace − work requiring long hours of stillness, without or with little mobility. The most common diseases resulting from occupational overload syndromes associated with the maintenance of machines are [8, pg.7]: − Anterior cruciate ligament injury, − injury of muscles and tendons, − osteonecrosis of the wrist, − fatigue fracture of the wrist (during work done by the repeated twisting the wrist), − de Quervain syndrome (occurs during thumb abduction with ulnar abduction of the wrist - compression and torsion) − inflammation of the tendons of the arms (resulting from the long-term maintaining of the arms raised above shoulder level), − neck and wrist muscle tension syndrome (characteristic of assembling parts). The human upper limb creates a bio-kinetic chain, which moves in the shoulder joint in three axes, performing bending, straightening, abduction, adduction, and medial and lateral rotation movements [21, pg.4]. When working with powered hand tools in a non-formalized environment, such as occurs, among others, during maintenance of machinery, there is a creation of an unfavorable layout of the biomechanical chain. In most cases, ailments and disorders of the musculoskeletal system lead to a short-term inability to work, but if the symptoms are left unchecked or are not related to the way of performing work irreversible effects may develop. An additional aspect of overload syndromes are negative economic effects resulting from work absences because of sickness and the medical treatment of workers, whose way of performing work has lead to overload injuries. For injuries resulting from the insufficient ergonomic quality of hand tools, included should also be those arising from too much pressure on the body (arm) of

3

the user. This force, combined with long-term effects, can strengthen the causes of diseases of the CTD group. On the basis of research, the PPT (Pressure-Pain Threshold) indicator is specified, which is known as “The amount of applied pressure which cannot be tolerated when applied to the hand.” The figure below classifies the optimal loads depending on the size of force, part of the tool user’s hand, and gender.

Women Men

mean PPT [kPa]

Fig. 2. Mean force of pressure of the tool on the user's hand (created based on [23])

From the above it results that the use of powered hand tools has a broad impact on the bodies of the operators, and the degree of impact depends on a number of interrelated factors [18]. Factors affecting the ergonomic quality of powered hand tools To prevent the consequences of the use of powered hand tools, their ergonomic quality should be shaped appropriately. Unquestionably the most important criteria for powered hand tools are those that are responsible for the trauma safety of workers. Presented below are ergonomic requirements designed to ensure the safety of machines that are carried and/or lead by hand. These criteria are divided into five groups, presented in Table No. 1.

4

Table 1. Ergonomic requirements for machines that are carried and/or lead by hand [29, pg. 186-190] Lp. 1.1.

1.2.

Content of Requirements Elements of the handgrip, in particular the handle, should be shaped and arranged so that the position of the axis of the device and its movements are readily felt in the kinetic sense ("deep feeling"). Elements of the handgrip, in particular the handle, should limit the number of degrees of freedom of mobile machines to only those that are necessary to perform the basic technological functions.

1.3.

The impact of energy supply wires (electrical cables, flexible hoses) must be reduced to the highest possible technical level.

2.1.

The location of the handgrip elements, in relation to the direction of the active force, should be such that the sum of the forces and torques weighing down the operator’s body is minimized.

5

Sketch

Lp. 2.2.

Content of Requirements The location of handgrip elements, in relation to the center of gravity of the mobile equipment, should be such that the sum of the forces and torques weighing down the operator’s body is minimized.

2.3.

The way of moving the active forces through the operator’s body to the ground and/or a support and the direction of reaction of the ground/support should be defined.

3.1.

A serial transmission of force load by the following groups of muscles should be avoided, and their number and/or weight should be reduced to a minimum.

3.2.

The forces necessary to maintain, manipulate, and steer a mobile machine during its operation and handling should be applied only to structurally adapted for this purpose handgrip elements.

3.3.

Handgrip elements should be shaped so that the forces weighing on the body are not transferred through friction.

3.4.

Handgrip elements should be shaped and arranged in accordance with the requirements of anatomy and physiology.

6

Sketch

Lp. 3.5.

Content of Requirements The maximum pressure on the skin of the operator should not exceed the limit values.

4.1.

When the purpose of the machine is to carry out multiple, repetitive cycles the controlling action should take place once and be in a two-state form (on off).

4.2.

Control operations of the flow of the energy flux supplied to the machine (electric energy, pressure, etc.) should not impair the certainty of grip and the precision of a mobile machine.

4.3.

Where there is a likelihood of disruption of visual observation (darkness, smoke, cloudy water, etc.) ability for tactile perception of control elements and their positions should be made.

5.1.

In the event of mechanical overload a moving machine should automatically lose contact with the operator (ex. detach from the hand), without causing a situation threatening of injury.

5.2.

The distance between the immobile parts of the machine (in particular the handgrip elements) and the moving parts cannot be reduced during the machine’s operation.

7

Sketch

Lp. 5.3.

Content of Requirements Elements controlling the inflow of external energy should be designed so that in case of loss of control by the operator of the situation (loss of balance, fainting, etc.) there is an automatic, permanent deactivation of power (and work activities).

5.4.

Stopping the flow of external energy cannot create a situation with a threat of injury.

Sketch

From the guidelines for designing powered hand tools designated are the maximum forces with which the tool user has to press the trigger of the mechanism starting operation of the machine: 10 N for a latch turned on by one-finger, 20 N for one which activation occurs through the index and middle fingers, and in the case of a button activated by four fingers the force should not exceed 30 N [5, pg.15]. A common postulate in the case of powered hand tools is the use of triggers that allow the tools to be turned on with at least two fingers. A powered tool that is designed in an optimal form for use in one direction may be less optimal for use in another direction. For the tools with a pistol handle, the angle between the handle’s axis and the working part of the tool contains most commonly between 100-110 °. The angle is optimal if the tool is mainly used on vertical surfaces, and the work is done with the tool held at or below the level of the elbow. If the tool is often held above the elbow, or if it is often used on horizontal surfaces, then the angle of 90 ° is more convenient. In addition, for tools that are normally used below the level of the elbow on horizontal surfaces, the angle between the handle and working parts should not exceed 110 ° [5, pg.17].

8

Fig. 3. Powered hand tools with various handle angles: 110 °, 90 °, 110 °, 90 ° and 180 ° [5, pg.17]

Presented below is a different form of dividing the risks arising from the use of powered hand tools, and after the dash, methods of their prevention [12]: − vibrations - anti-vibration system, − mechanical noise - sound-proof enclosure, − aerodynamic noise - air exhaust muffler, − physical burden of tool gripping – auto-control of gripping force n