Tractor Design & Testing 3(2+1)

Lesson 21. Importance of ergonomics in tractor and agricultural machinery design.

2. Ergonomics in Tractor and agricultural machinery design

The ergonomic aspects during application in agricultural machinery are of great importance as the operator has to operate the machine in field. The physiological as well as psychological fatigue affects performance of the operator. Objective of ergonomics is not only to improve work performance but also to improve human comfort as well as safety. If ergonomic aspects are not given due consideration, the performance of the system will be poor and the effective working time will be reduced. The goal of ergonomics is to design workplace to conform to the physiological, psychological, and behavioural capabilities of workers. There are many factors acting as stress on the operator during the work. These stresses may be due to workload, immobilization for longer duration work, ambient temperature, relative humidity, vibrations, noise, dust, smoke and other gases. A feeling of chance of accident during work, space confinement, overload of information to be handled, etc. results in psychological fatigue. During the ergonomic studies, these stresses can be measured in terms of strain on the operator. The most important among physiological strains are related to heart activity, respiration, discomfort, muscular fatigue, etc. For the psychological /mental strain measurements stress on eyes, hearing loss, errors, speed of work, work performance are mostly used in studies (Fig.21.1).

3. Subjects 

Whenever we have to plan for ergonomic study, subjects (operators or workers) are an integral part of the study. The objective is to eliminate the affect of individual characteristics from the rest of independent parameters and also to have an estimate of human workload during the actual work. The subjects must be medically fit and represent real user population in operation of the selected machinery. Their selection is made on the basis of gender, age and weight. Generally male subjects are selected for conducting ergonomic studies on agricultural machinery in India.

4. Physiological factors for measurements

Physical activities stimulate certain physiological responses in human beings. These responses provide basis for human energy expenditure and fatigue. The physiological measurements are made generally in terms of heart and respiration activities.

 
Fig.21.1. Stresses and strains in ergonomic studies

5. Heart rate

Heart rate (HR) is the most reliable dependent parameter in ergonomic studies. This is because the heart rate has a direct and linear relationship with the human workload and stress. A starting period of 2-3 minutes is sufficient for heart/pulse rate to stabilize depending upon nature of exercise. Also, care has to be taken so that the operator is not subjected to workload leading to heart rate more than HR_max i.e. the upper limit of heart rate allowed during an activity.

Here,

HR_max = 220 – age                                                                                          ...(21.1)

6. Respiration rate

The respiration is another basic variable in work physiology as it is linearly related to the workload. It is measured in terms of rate of volume of air inhaled or air exhaled or oxygen intake (VO₂) or respiration rate. The greater the demands made on the muscle by the physical activities, the more air or oxygen is inhaled. The human energy expenditure is computed by multiplying the oxygen consumption with the calorific value of oxygen. The human workload has been categorized between light work and extremely heavy work depending upon heart rate or oxygen consumption (Table.21.1).

Table 21.1. Limits of physiological responses and work category.

S. No.	Work category	Physiological response
		Oxygen consumption (l/min)	Heart rate (beats/min)
1	Light work	< 0.5	Up to 90
2	Moderate work	0.5 - 1.0	90-110
3	Heavy work	1.0-1.5	110-130
4	Very Heavy work	1.5-2.0	130-150
5	Extremely heavy work	> 2.0	150-170

 

7. Discomfort rating

Body posture is one of the major factor which causes muscular fatigue and discomfort in the body. Uncomfortable body posture in different activities reduces work efficiency, capacity and safety of operator. It is widely agreed that awkward working postures are the principle risk factor associated with muscular-skeletal injuries and disorders during occupational activities. The effect due to working posture can be measured in terms of overall discomfort rate and body part discomfort rate techniques. Table 21.2 gives the pain intensity score as measure of overall discomfort rate. The same score can be used for measurement of body parts discomfort rating.

Table 21.2. Pain intensity score as a measure of overall discomfort rating (ODR).

Subjective feeling	ODR Score	Subjective feeling	ODR Score
Comfortable	0	Moderately painful	4
Uncomfortable	1	Highly painful	5-6
Pain starts	2	Very highly painful	7-9
Slightly painful	3	Extremely painful	10

 

Psychological factors for measurements

Certain working operations need overload of information, vigilance, danger or work performance accuracy. In such conditions, there is a higher load on sensory/neuro organs of the human body. Such situations cause higher stress on eyes, hearing system, brain activity, etc. resulting in poor work performance, more errors, more missings, lesser endurance, etc. These situations sometimes may lead to headache, drowsiness and even accident. These stresses can be measured in terms of blink rate, flicker fusion, critical hearing threshold level, speed & errors in work performance, etc.

1. Anthropometry

Anthropometry is the technology of measuring various human physical traits, such as body dimensions of workers and their strength. Engineering Anthropometry is an effort to apply such data to equipment and workplace design to enhance the efficiency, safety and comfort of the operator. Anthropometric measures vary considerably due to effect of various factors viz. gender, race, age, etc. The anticipated user population, therefore, controls the application of anthropometric data. An anthropometric database, involving 79 body dimensions and 16 strength parameters, of agricultural workers is being prepared at the country level. It is critical for the designer to consider the anthropometry at the time of conception of the design rather than as a follow–up or add-on part of the design. The other application of such database is workspace and controls layout for the machinery. Table 21.3 lists a few important body dimensions and their usefulness. However, the database is to be given statistical application in terms of range, mean, standard deviation, 5^th and 95^th percentile values.

Table 21.3. Some important anthropometric dimensions and their usefulness.

S.N.	Dimension	Definition	Usefulness
1.	Weight	It is measured on a calibrated weighing scale.	General body description.
2.	Stature	The vertical distance from standing surface to the top of the head. The subject stands erect and looks straight-forward.	General body description, work place designs.
3.	Vertical reach	The vertical distance from standing surface to the height of middle finger when arm hand and fingers are extended vertically.	Workplace layout, design of controls
4.	Eye height	The vertical distance from standing surface to the inner corner of the eye. The subject stands erect and looks straight forward.	Design of controls and displays
5.	Knee height	The vertical distance from standing surface to the midpoint of knee cap. The subject stands erect and looks straight forward.	Body linkages, work place design.
6.	Arm reach from wall	The distance from the wall to the tip of middle finger measured with subject shoulder against the wall, his hand and arm extended forward.	Design of controls and display panel, work place layout.
7.	Sitting height	The height, from the sitting surface, to the top of the head. The subject stands erect and looks straight forward.	General body description, control panel layout, work place layout.
8.	Sitting eye height	The height from the sitting surface to the external canthus. The subject sits erect and looks straight forward.	Display and control panel design, seat design, visual field determination.
9.	Knee height sitting	The height from the footrest surface of the musculature just above the knee. The subject sits erect and looks straight forward.	Work place layout, seat design, general body description.
10.	Hand length	The distance from the base of the hand to the top of the middle finger measured along the long axis of the hand.	Handle design, control panel design, hand tool design.
11.	Span	The distance between the tips of right and left middle fingers when the subject’s arms are maximally extended laterally.	Work place design, design of controls.