Tractor Design & Testing

Lesson 23. Tractor noise, vibration and other environmental factors

2. Noise

The noise produced by engines may cause discomfort, nervousness, tension, irritability and fatigue. Levels from 86 to 115 dB(A) can cause specific effects to the ear such as the damage of the corticells and can involve psychosomatic diseases. Noise also results in increase in the pulse rate & blood pressure and irregularities in heart rhythm. Occupational Safety & Health Administration, USA has given a standard OSHA-1910.95 for occupational noise exposure. It mentions that the permissible daily (8-h) exposure to the operator is to be upto 90 dB (Table 23.1). The other remedial measures against noise as health hazard may be noise reduction at source or at emission or use of personal protection such as ear plugs.

Table 23.1. Permissible daily noise exposure as per OSHA -1910.95.

Duration per day, hours	Sound level, dB(A)	Duration per day, hours	Sound level, dB(A)
8	90	1.5	102
6	92	1.0	105
4	95	0.5	110
3	97	0.25	115
2	100	--	> 115

 
i. Operator exposure to noise

Zander (1972) stated that the application of human biological sciences to achieve the optimum mutual adjustment of man and work, the benefits being measured in terms of human efficiency and well being. Human factors when properly incorporated in design, allow the operator to perform task with better efficiency, comfort and safety. In tractor design, noise is one of the important factors which need to be considered, as the design of tractor operator work place has direct impact on noise levels at operators’ ear level. Noise is unwanted sound and its effect depends on frequency, sound pressure level and duration of exposure. Noise in tractor is generated due to rotating and reciprocating parts, vibration in engine components, exhaust gases and flow of hydraulic fluids. It is transmitted by the moving (rotating, reciprocating and vibrating) structural components of tractor or air borne or combination of the two. The tractors generate sound in the range of 125-500 Hz depending upon age and ear sensitivity. Hence, continued exposure of tractor noise to its operator will have harmful effects on their comforts and efficiency. Noise has annoying and damaging effect. Therefore, the design of mechanical components of tractor and its engine to limit the sound energy levels for operator’s comfort and efficiency is a challenge for the design engineers.       

Noise: It is defined as acoustic sound or unwanted sound and is wasted energy damaging to human ear which is physically speaking, mechanical vibrations in gaseous, fluid or solid medium. It is characterized by frequency, amplitude and phase. The simplest vibration is a pure tone that consists of sinusoid with frequency

The frequency is given by

                          ... (23.1)

Where,

T = period in seconds.

The magnitude of noise is usually expressed in terms of RMS (root mean square) value, which is given by

                                            ... (23.2)

It is measured with the help of noise level or decibel meter has got three frequency weighing scales A, B and C. It is expressed in terms of dB(A). The sound level meter incorporates electrical circuits known as weighing networks. A, B, C scale operating conditions can be selected on the sound level meter. Least sensitivity is provided in A-scale and highest in C-scale. A-scale is used for measuring frequencies that human ear can detect.

Sound Pressure Level (SPL): It is expressed as sound level (SPL) relative to reference sound pressure.

                                                ... (23.3)

And                                                                                      ... (23.4)

Where,

P  = measured RMS sound pressure, N/m²

P₀ = reference sound pressure, N/m²

log = logarithm to the base 10

Sources and effect of noise on tractor operator: According to Nebraska’s Organization for Economic Cooperation and Development (OECD). Tractor tests the tractor noise level at the operator’s site will be in the range of 89-98 dB (A). Almost all the models of tractors manufactured n India also produce noise level in the range of 88-100 dB (A). Noise levels of farm machinery ranged from 90 to 97 dB (A) for the operations normally carried out by the operators.
 
Noise sources in tractor: One of the major problems associated with the tractor is the high level of noise emission. Main sources of noise in tractors given under different categories are as:

Tractor engine: High noise generating components in tractor engine are:

(a)Engine exhausts                                                 (b) Intake
(c) Combustion                                                     (d) Piston slap
(e) Fuel injection pump                                          (f) Valve train
(g) Gears                                                              (h) Bearings

(i) Crank shaft

• Hydraulic system
• Differential (power transmission system)
• Brakes etc.

Out of the above noise sources in a tractor, the noises produced by combustion and piston slap are the biggest sources of noise. It is mainly caused due to pressure pulsation and turbulent flow created by sudden opening and closing of inlet and exhaust valves. The tractor operators in India are exposed to noise levels above the safe permissible limit drawn by Occupational Safety and Health Agency (OSHA), ILO and BIS in 90% of models of all category of tractors.

Levels of noise of different sources: The generation and transmission of noise by various sources depend on the frequency and level of noise. It has been reported that highest sound levels on tractors were in the frequency range of 75 to 300 Hz (Weston 1963). Noise levels of different sources are given in Table 23.2.

Reference air borne sound pressure level (SPL) = 0.00002 N/m² = 0 dB

The noise conservation measures are mandatory at 65 dB (A) and noise controls are required when noise levels reach 90 dB (A) for 8 hours a day.

Principal effects of noise: Communication between man and his tasks (or men) takes place through hearing. Human ear converts the sound wave into neural signals which can be transmitted to brain. Exposure to noise may adversely affect overall efficiency, Safety and hearing ability of the workers. The entire spectrum of audible noise range from 20 Hz to 20 kHz and deterioration in threshold of hearing (DITH) was observed in all the audible frequencies if close of noise is continuous for longer period of time. The noise generated is beyond tolerable limits. OSHA in USA has set permissible noise exposure level of 90 dB (A) to 8 hours per day. The average working hours per day of farm machinery operators vary from 12-14 in sowing and harvesting season and 5-6 h in lean season. The exposure to noise in sowing/ harvesting season is sufficient to cause noise induced hearing loss (NIHL).

Table 23.2: Different sources and levels of noise

Sr. No.	Source	Levels
1.	Modern Tractor with cab	80 dB (at operator’s ear)
2	Tractors without cab	89-98 dB (at operator’s ear)
3.	Threshers and Combine harvesters	90-98 dB
4	Power operated spray pumps	90-98 dB
5	Power Tiller operator zone	105-110 dB
6	Milling machine (1.2 m)	86 dB
7	Cotton ginning Machines	96 dB
8	Power lawn mover	98 dB
9	Pneumatic chipper (2m)	113 dB

The noise environment and its severity associated with the tractors may have following main effects on the operators:

• Undesired physiological reactions as noise is annoying (objectionable to unbearable).

• Lowering of concentration, fatigue due to longer exposure and interference which decreases the speed, quality and precision of work. Longer exposure to noise may affect the performance in terms of rhythm disturbance and interference with worker to worker communication in a team.

• Hearing loss due to continuous and prolonged exposure to high noise levels.

• Damage to hearing caused by noise (temporary and permanent) due to hearing thresh hold shift. Excessive noise would cause more strain on operator and may result in permanent hearing loss. They experience stress related disorders and suffer temporary or permanent shifts to hearing ability. This is due to deterioration in threshold of hearing (DITH).

• Heart rate of tractor operator increase with increased exposure of noise.

• Greater incidence of physical illness and increased strain among those exposed to high intensity noise. Evidence of pathological effect of continuous exposure of 95 dB (A) or more was reported by some researchers.

3. Vibration in tractor

The vibration is defined as oscillatory motion about a fixed point. It is defined usually by its frequency, amplitude, velocity, acceleration and direction. However, the human vibrations are measured in terms of acceleration only. Mechanical vibrations have instantaneous and long term effects upon the human body. Kinds of effect depend upon the duration of exposure and the frequency of vibrations. In walk behind type machine, vibrations are transmitted to the operator through his hands. In case of a riding type, vibrations are transmitted to the operator through seat as whole body vibration (WBV) and through his hands as hand transmitted vibrations (HTV). Workers exposed to hand arm vibrations often experience aches and pains in upper limbs. Daily exposure to hand arm vibrations over a period can cause permanent physical damage known as "White Finger Syndrome” or Reynaud’s phenomenon of occupational origin. Exposure to whole body vibrations can cause back injury. Hence measurement and evaluation of human vibrations are necessary for assessing operator’s comfort and to suggest remedial measures for continuous operation of machine. The vibration studies are to be conducted as per standard guidelines. The ISO 5349-1 & 2: 2001; ISO 2631-1: 1997; ISO 9996: 1996; ISO 5805: 1997; ISO 13090-1: 1998; ISO 10816-6; BS 6841: 1987; IS 13548: 1992 are used in vibration studies.

I. Human exposure to vibrations: Any engineering system possessing mass and elasticity are capable of relative motion. When this motion repeats after a given interval of time, then this motion is known as vibration. In many cases it is undesirable and is a form of wasted energy. This is particularly true in tractors and farm machinery as they generate noise, break down parts and transmit unwanted forces and movements to different components. Vibrations and noise in tractor are generated due to rotating and reciprocating parts, vibrations in engine components, exhaust gases and flow of hydraulic fluids. These are transmitted by the moving (rotating, reciprocating and vibrating) structural components of tractor of air borne or combination of the two. The physical objects have their individual resonant frequencies which are a function of their mass. Tractors with pneumatic tyres have resonant frequencies primarily in the range of 1 to 4 Hz and for track type tractors more than 4Hz. It is important to note that in the frequency range of 4 to 8-Hz, human tolerance to vibration is minimum. Exposure of human body to vibrations can result in

• Biological effects

• Mechanical effects

• Psychological effects

II.Vibration: It can be defined as an oscillatory motion of mechanical system and can generate sound in a gas, fluid, or solid medium. Vibration is characterized by frequency, amplitude and phase. In the simplest case mechanical vibration can be derived from simple harmonic wave form. In vibration analysis, the frequency of periodic motion is number of cycles per unit time and it is a function of time period as given below.

Where,

T= period in time required for a periodic motion to repeat itself in seconds

The frequency of vibration can be sinusoidal, complex or random. It is important to note that in the frequency range of 4 to 8Hz, human tolerance to vibration is a minimum.

III. Types of Vibrations

The vibration in a system can be of following two types:

Forced vibration: It is defined as the periodic motion observed, as the system is displaced from its static equilibrium position. The forces acting are spring force, the friction force, the weight of the vibrating mass and external forces acting on the system during its vibratory motion. The system will tend to vibrate at its natural frequency at forced vibration. This will follow the frequency of the excitation force. In the presence of friction, the portion of motion not sustained by sinusoidal excitation force will gradually die out. As a result of this the system will vibrate at the frequency of excitation force regardless of initial conditions or natural frequency of the system. The part of sustained vibration is called as steady state vibration or response of the system.

Free vibration: It is defined as the periodic motion observed as the system is displaced from its static equilibrium position. The forces acting are spring force, the friction force and the weight of the vibrating mass. This may occur when after an initial disturbance, no external force is present i.e. F (t) =0. The vibration will diminish with time due to pressure of friction. This is sometimes called as transient vibration. The differential equation for free vibration would be as under:

iv. Vibration in tractor chassis: The tractor driving may have considerable ill-effects on the health of operator. This is largely due to effect of vibrations and shock continuously acting on human body and setting up harmful stamina and partly to the need to keep the body in cramped condition and unhealthy posture for long period. Tractor ride vibration intensities are normally related with the ground speed. The vibrations become intolerable as the speed is increased. Tractors with pneumatic tyres have resonant frequencies primarily in the range of 1 to 4 Hz and for track type tractors more than 4Hz. It is important to note that human tolerance to vibration is minimum in the frequency range of 4 to 8 Hz, According to survey majority of large tractor are found to be operating at less than two-third of maximum power because of following reasons.

• Operator’s inability or unwillingness to withstand the ride at full speed.

• Ride induced vibrations do have ill-effects on the operators.

Therefore, proper care must be taken in designing tractors particularly their seat.

• To reduce vibrations and shocks to a minimum by appropriate suspension and shock absorption.

• To arrange the tractor controls in a manner to a comfortable posture and minimum effort.

Vibration input to tractor seat due to vertical motion of tractor body as tractor ride vibration intensities are positively related with ground surface and ground speed during field operations is given by following equation:

 
                 ... (23.5)

Where, Zi           = Vertical motion input to tractor seat, m

            Zt        = Vertical motion of tractor e.g., m

            θ         = Rotation or pitch about tractor e.g, radians

            g         = Longitudinal distance from tractor seat to c.g, m

4. Tractor Seat Design

From vibration point of view: Tractor seat suspension for vertical vibrations transmitted by the tractor chassis to the seat and experienced by the tractor operator. During the process of whole body vibration of tractor operator, the vibration transmitted to the body can be amplified or attenuated due to body posture, type of seat and frequency of vibration. The dynamic performance of seat suspension can be analyzed based on output vibration in response to a given input vibration, which can be expressed as a function of

-          frequency of input vibration

-          seat spring rate and

-          mass damping properties

From eqn. 23.6, it is clear that ω (undamped natural frequency) of the seat is a vital Parameter for matching with vibration input from the tractor. The value of ω (undamped natural frequency) of the seat should be less than the frequency input from the tractor chassis.

Therefore,

to avoid vibration attenuation in the seat suspension. A soft spring should be used to keep the natural frequency value to a minimum. This will also affect ease of adjustment and convenience of the operator.

Also                                           ... (23.6)

Where,   M = mass of tractor seat and tractor operator, kg

ω_n= undamped natural frequency of tractor seat, rad/s

It has been observed that the seat suspensions used on tractors and agricultural machines would normally require a damping rate © of about 30-50% of critical damping rate (C_c). Damping can be controlled by appropriate seat design, spring design and cushion of seat. Thus, the physical effects of vibration on whole body of operator are greatly influenced by body posture, and materials provided between the body and source of vibration i.e. seat, cushion and springs etc.

A properly designed seat can effectively reduce the vibration intensity which would otherwise be transmitted to operator, especially in low frequency range (2 to 5 H_z) and may have considerable ill-effects on the health of operator. The tractor seat suspensions are comprised of cushions mounted to resilient spring/damper to reduce the level of input tractor chassis vibrations.

5. Effect of whole body vibrations on operations

Exposure of human body to vibration can result in Biological, Mechanical, Physiological and Psychological effects as given below:

1. There may be direct mechanical interference with the activity due to whole body vibrations.

2. The whole body vibrations may result in increased working stress level, tension and deformation as a source of localized pain and fatigue.

3. General discomfort and anxiety with certain subjective sensations which may be annoying to the operators.

4. Abdominal, chest and testicular pains which may be contributing to operator’s long term health.

5. There may be indirect physiological induced alterations/effects on metabolism.

6. The combined mechanical, physiological and physiological responses to sinusoidal vibrations can affect the operator’s ability to perform control functions in different types of tasks as under:

(i) Visual tasks: dial reading, visual scanning

(ii) Simple motor task that require precision and coordination

(iii) Psychomotor tasks including tracking tasks

7. Fatigue in the body.

6. Dust

Dust consists of tiny solid particles carried by air currents. Agricultural activities like field preparation, harvesting and threshing produce a lot of dust. Larger particles settle down while lighter ones remain suspended in the air.Respirable dust (size 0.1 to 5 microns) penetrates the respiratory system and settles deep into the lungs; thus acting as serious health hazard. Personal dust protecting masks, having plastic net, tissue paper, cloth, foam, etc. as filtering material, gives sufficient protection against respirable dust.