Wednesday, August 15, 2012

IE Research Paper Summary


Understanding the link between psychosocial work stressors and work-related musculoskeletal complaints

Erin M. Eatough, Jason D. Way, Chu-Hsiang Chang
     

 Applied Ergonomics,         
 May 2012, Pages 554–563

                   

Objective

The goal of the current study is to test a stress-based model that links psychosocial work stressors, strain, and WRMSD symptoms.

Summary

It is well established that psychosocial work stressors relate to employees’ work-related musculoskeletal disorder (WRMSD) symptoms. Work-related musculoskeletal disorders (WRMSDs) affect tendons, tendons sheaths, muscles, nerves, bursae, and blood vessels in the body. Every year, more than 70 million physician office visits can be attributed to WRMSD-related complaints the economic burden resulting from symptoms related to WRMSDs (including costs associated with workers’ compensation, lost wages, and productivity) at $50 billion annually. Work-related musculoskeletal problems therefore represent a significant threat to employees’ health and wellbeing across a wide range of industries and occupations although multiple theoretical models (e.g., Bonger et al., 1993; Faucett, 2005; Sauter and Swanson, 1996) exist in speculating the mechanisms underlying the associations between psychological factors and WRMSDs, research remains inconsistent in supporting hypotheses generated by different models (e.g., Swanson and Sauter, 2006; Wademan and Kjellberg,2007). These conflicting research findings may be partly due to the lack of precision in the definition and measurement of the psychosocial aspects of jobs.We will first introduce the occupational stress model and explain how psychosocial work stressors are related to musculoskeletal complaints through psychological strain Specific hypotheses based on the model will be presented. Results from structural equation modeling will be used test the proposed hypotheses.
Existing literature supports that these psychosocial work stressors have significant relationships to employee strain responses (e.g., Jackson and Schuler, 1985; Spector and Jex, 1998). For example, empirical studies have demonstrated that role conflict, job control and leadership are associated with strain (e.g., Jex and Beehr, 1991; Siu et al., 2004; Spector, 1986; Spector and Jex, 1998). Role conflict has been shown to relate to employee strain responses (e.g., Jacksonand Schuler, 1985; Spector and Jex,1998). Similarly, control has been shown to relate to strain (Spector and Jex,1998). Effective leadership. one proposed model by Sauter and Swanson (1996), an ecological model of musculo- skeletal disorders, is based on the notion that both physical and psychological factors in the work environment contribute to the experience of WRMSDs. While there are many possible mediating mechanisms between strain and WRMSDs as described above, we did not test them directly in the current study. Rather, the current study will test a theoretical model that links stressors to work-based musculoskeletal complaints via psychological strain
Hypothesis1. (a) There will be a positive relationship between role conflict and work-related musculoskeletal complaints and (b) this relationship will be mediated by psychological strain. Hypothesis2. (a) There will be a negative relationship between job control and work-related musculoskeletal complaints and (b) this relationship will be mediated by psychological strain. Hypothesis3. (a) There will be a negative relationship between safety-specific leadership and work related musculoskeletal complaints and (b) this relationship will be mediated by psychological strain

Methods: Data were obtained from 277 full-time employees. The majority of the participants were females (79%) and Caucasian (69%), or African American (10%). The average age of the participants was 24 years old (SD ¼ 6.6). Participants had an average tenure of 3 years (SD ¼ 3.5) in their present job and worked a minimum of 20 h per week.

Measures: Demographic variables, physical job demands, Safety-specific leadership, Autonomy/control, Role conflict, Anger, anxiety, and depression, Frustration, Musculoskeletal complaints
Data Analysis: For the exogenous variables (i.e., role conflict, control, and safety leadership), scale items were used as indicators for the latent

Factors: For psychological strain, the scale scores of anger, anxiety, depression, and frustration were used as indicators. The structural equation model was tested using the TCALIS procedure in SAS 9.2 (Statistical Analysis Software) using maximum likelihood estimation

Discussion:  purpose of the current study was to understand the link between psychosocial work stressors, strain, and the musculoskeletal symptoms in a stress processed-based model. By cleanly separating stressors from strains, relying on improved measures, and using sophisticated methodology to test the theoretical model, the current work provides a significant added value to the current literature in this area. Our methodology included mediation analyses using structural equation modeling while ensuring common method variance alone was not responsible for construct covariance. Our results suggested that high levels of various psychosocial work stressors (namely low safety leadership, low job control, and high role conflict) were associated with increased strain. Strain, in turn, was related to higher levels of WRMSD symptoms of the wrist/hand, shoulders, and lower back. These results were consistent when controlling for the physical demands of the job. The fact that self-reports of physical demands were not related to any of the physical symptom reports suggests that it may not be any physical demands of the job causing the symptoms, but rather the psychological work stressors and their resulting emotional strain. Furthermore, evidence of a partial mediation by strain between control and both wrist/hand symptoms and shoulder symptoms was found as well as a partial mediation between safety leadership and wrist/hand symptoms. These partial mediation effects suggest that there may be additional explanations regarding the mechanisms linking control and safety leadership to WRMSD symptoms. The results of the SEM model demonstrate that psychosocial stressors in the work environment can have meaningful links to employee health on both psychological and physical levels.

Results: reports the means, standard deviations, internal consistencies, and correlations among the focal variables. Each psychosocial work stressor was significantly related to at least one of the WRMSD symptoms. Consistent with our hypotheses, safety leadership was significantly related to wrist/hand (r ¼ _.14) and lower back symptoms (r ¼_.13), and role conflict was significantly related to lower back symptoms (r ¼ .16), thus providing partial support for Hypotheses 1a and 3a. Interestingly, control had positive, significant relationships with shoulder (r ¼.14) and wrist/hand symptoms (r ¼ .12), which was opposite from our expectation. Thus, Hypothesis 2a was not supported.
1 Model testing, 2 model parameters estimates.

Theoretical and practical implications: Role conflict, job control and safety-specific leadership all had significant path coefficients to strain. In line with previous literature, role conflict was associated with increased levels of psychological strain. This supports the notion that inconsistent demands from multiple sources (i.e., multiple supervisors) can have a significant impact on employee psychological well-being. Furthermore, in line with previous work, job control was significantly related to strain such that lower levels of control were associated with higher levels of strain (Karasek, 1979). Finally, safety-specific leadership had a significant association with strain. This finding underscores a lack of safety-specific leadership as an occupational stressor that may elicit psychological distress in employees. Further, strain fully mediated the association of role conflict with WRMSD symptoms, suggesting that the psychological states arising from role conflict are related to increases in WRMSD complaints. Our results suggest that low levels of role conflict may be associated with reduced levels of strain which in turn leads to fewer WRMSD complaints. This is in line Sauter and Swanson’s (1996) ecological model of musculoskeletal disorders, suggesting that psychosocial work stressors contribute to higher reports of WRMSDs through their effects on psychological strain. This finding is also consistent with previous work demonstrating strain as an important precursor of musculoskeletal symptoms (Lim and Carayon, 1993) and emphasizes the role of psychological distress in understanding how role conflict in one’s job may contribute to poorer physical health.
Conclusion: This study demonstrates that high levels of psychosocial work stressors (high role conflict, low job control, and low safety-specific leadership) are associated with increased employee strain. Strain, in turn, related to higher levels of WRMSD
symptoms of the wrist/hand, shoulders and lower back. Partial mediation of some relationships was also found suggesting other explanations for the relationships are plausible. This work supports the notion that the psychosocial components of the work environment have important links to employee health, as assessed by WRMSDs. To maintain a healthy and productive workplace, organizations should work to reduce psychosocial work stressors which could result in high levels of strain and in turn, physical complaints in employees.

INDUSTRIAL ENGINEERING ASSIGNMENT

     Industrial Engineering Assignment - Component design


A project Report
On
Design of Connecting rod for an Automobile industry
 
 
Under the guidance of
    Prof K.V.S.S Narayan Rao
                                                                                 Prepared by: 
                                                                                 Samay Singh Meena (Roll No 86)
                                                                                 Promod Kumar Maharana (Roll No 64)
                                                                                 PGDIE-42
                                                                                 Sec-B
                                                             Content    
                                         
1.             Introduction                                                                                                                   
2.            Functioning of connecting rod                                                                                       
3.            Design process flow of connecting rod.                                                                  
4.            Manufacturing process flow of connecting rod.                    
5.            Conclusion                                                                          
6.            References                                                                       
           
      
           Introduction                                                    
In a reciprocating piston engine, the connecting rod or conrod connects the piston to the crank or crankshaft. Together with the crank, they form a simple mechanism that converts linear
motion into rotating motion. Connecting rods may also convert rotating motion into linear motion.
Historically, before   the development of   engines, they were first used in this way.
As a connecting rod is rigid, it may transmit either a push or a pull and so the rod may rotate the crank through both halves of a revolution, i.e. piston pushing and piston pulling. Earlier mechanisms, such as chains, could only pull. In a few two-stroke engines, the connecting rod is only required to push.
Today, connecting rods are best known through their use in internal combustion piston engines, such as car engines. These are of a distinctly different design from earlier forms of connecting rods, used in steam engines and  steam locomotives.
         Functioning of connecting rod

         Design steps of Connecting rod.
                                    
                                                                                         
  Need or Aim:
   The Primary purpose of connecting rod is to transmit gas forces applied on the piston due to      ignition to crankpin of crankshaft assembly.
                                        
   Synthesis(Mechanism)
                                                    
    The above mechanism will give the required motion to the connecting rod.
                                        
   Analysis of Forces in connecting rod.
    The various forces acting on the connecting rod are as follows:
    1. Force on the piston due to gas pressure and inertia of the reciprocating parts,
    2. Force due to inertia of the connecting rod or inertia bending forces,
    3. Force due to friction of the piston rings and of the piston,       
    4. Force due to friction of the piston pin bearing and the crankpin bearing.
1.  Force on the piston due to gas pressure and inertia of reciprocating parts
Consider a connecting rod PC as shown in Fig
                                        
Forces on the connecting rod
Let p = Maximum pressure of gas,
D = Diameter of piston,
A = Cross-section area of piston = (π /4)D2
mR = Mass of reciprocating parts, = Mass of piston, gudgeon pin etc. + 1/3 rd mass of connecting rod,
ω = Angular speed of crank,
φ = Angle of inclination of the connecting rod with the line of stroke,
θ = Angle of inclination of the crank from top dead centre,
r = Radius of crank,
l = Length of connecting rod, and
n = Ratio of length of connecting rod to radius of crank = l / r.
We know that the force on the piston due to pressure of gas,
FL = Pressure × Area = p . A = p × (π /4)D2
and inertia force of reciprocating parts,
FI = Mass × *Acceleration = mR .ω2. r (cos n  θ + (cos 2 θ)/n)
It may be noted that the inertia force of reciprocating parts opposes the force on the piston when
it moves during its downward stroke (i. e. when the piston moves from the top dead centre to bottom
dead centre). On the other hand, the inertia force of the reciprocating parts helps the force on the piston when it moves from the bottom dead centre to top dead centre.
Net force acting on the piston or piston pin (or gudgeon pin or wrist pin),
               
FP = Force due to gas pressure   Inertia force = FL    FI
The –ve sign is used when piston moves from TDC to BDC and +ve sign is used when piston
moves from BDC to TDC.
When weight of the reciprocating parts (WR = mR . g) is to be taken into consideration, then
         FP = FLF1 ±WR
2. Force due to inertia of the connecting rod or inertia bending forces
Consider a connecting rod PC and a crank OC rotating with uniform angular velocity ω rad / s. In order to find the acceleration of various points on the connecting rod, draw the Klien’s acceleration diagram CQNO as shown in Fig. 32.11 (a). CO represents the acceleration of C towards O and NO represents the acceleration of P towards O. The acceleration of other points such as D, E, F and G etc.,on the connecting rod PC may be found by drawing horizontal lines from these points to intresect CN at d, e, f, and g respectively. Now dO, eO, fO and gO respresents the acceleration of D, E, F and G all towards O. The inertia force acting on each point will be as follows:
Inertia force at C = m × ω2 × CO
Inertia force at D = m × ω2 × dO
             Inertia force at E = m × ω2 × eO, and so on.                
       
                                                                   
Inertia force per unit length at the crankpin = m1 × ω2 r
and inertia force per unit length at the piston pin = 0
Inertia force due to small element of length dx at a distance x from the piston pin P,
    dF1 = m1 × ω2r ×x/l × dx
Resultant inertia force,=m /2 × ω2 r 
 
3. Force due to friction of piston rings and of the piston.
The frictional force ( F ) of the piston rings may be determined by using the following expression : 

F = π D · tR · nR · pR · μ
where D = Cylinder bore,
tR = Axial width of rings,
nR = Number of rings,
pR = Pressure of rings (0.025 to 0.04 N/mm2), and
μ = Coefficient of friction (about 0.1).
Since the frictional force of the piston rings is usually very small, therefore, it may be neglected.
The friction of the piston is produced by the normal component of the piston pressure which varies from
3 to 10 percent of the piston pressure. If the coefficient of friction is about 0.05 to 0.06, then the frictional force due to piston will be about 0.5 to 0.6 of the piston pressure, which is very low. Thus, the frictional
force due to piston is also neglected.
4. Force due to friction of the piston pin bearing and crankpin bearing
The force due to friction of the piston pin bearing and crankpin bearing, is to bend the connecting rod and to increase the compressive stress on the connecting rod due to the direct load. Thus, the maximum  compressive stress in the connecting rod will be
 σc (max) = Direct compressive stress + Maximum bending or whipping stress due to inertia bending stress.
      Material Selection.

The connecting rods are usually manufactured by drop forging process and it should have adequate strength, stiffness and minimum weight. The material mostly used for connecting rods varies from mild carbon steels (having 0.35 to 0.45 percent carbon) to alloy steels (chrome-nickel or chrome molybdenum steels). The carbon steel having 0.35 percent carbon has an ultimate tensile strength of about 650 MPa when properly heat treated and a carbon steel with 0.45 percent carbon has a ultimate tensile strength of 750 MPa. These steels are used for connecting rods of industrial engines. The alloy steels have an ultimate tensile strength of about 1050 MPa and are used for connecting rods of  aeroengines and automobile engines. 
                                                                                          
Design of Connecting Rod
In designing a connecting rod, the following dimensions are required to be determined :
1. Dimensions of cross-section of the connecting rod,
2. Dimensions of the crankpin at the big end and the piston pin at the small end,
3. Size of bolts for securing the big end cap, and
4. Thickness of the big end cap.
The procedure adopted in determining the above mentioned dimensions is discussed as below :
                                                                        
# Design a connecting rod for an I.C. engine running at 1800 r.p.m. and developing a maximum pressure of 3.15 N/mm2. The diameter of the piston is 100 mm ; mass of the reciprocating parts per cylinder 2.25 kg; length of connecting rod 380 mm; stroke of piston 190 mm and compression ratio 6 : 1. Take a factor of safety of 6 for the design. Take length to diameter ratio for big end bearing as 1.3 and small end bearing
as 2 and the corresponding bearing pressures as 10 N/mm2 and 15 N/mm2. The density of material of the rod may be taken as 8000 kg/m3 and the allowable stress in the bolts as 60 N/mm2 and in cap as 80 N/mm2. The rod is to be of I-section for which you can choose your own proportions. Draw a neat dimensioned sketch showing provision for lubrication. Use Rankine formula for which the numerator
constant may be taken as 320 N/mm2 and the denominator constant 1 / 7500.
Given : N = 1800 r.p.m. ; p = 3.15 N/mm2 ; D = 100 mm ; mR = 2.25 kg ; l = 380 mm
= 0.38 m ; Stroke = 190 mm ; *Compression ratio = 6 : 1 ; F. S. = 6.                                           
The connecting rod is designed as discussed below :
1. Dimension of I- section of the connecting rod
Let us consider an I-section of the connecting rod, as shown in Fig. 32.14 (a), with the following
proportions :
                                                                                            
Flange and web thickness of the section = t
Width of the section, B = 4t
and depth or height of the section, H = 5t
First of all, let us find whether the section chosen is satisfactory or not.
The connecting rod is considered like both ends hinged for buckling about X-axis and both ends fixed for buckling about Y-axis. The connecting rod should be equally strong in buckling about both the axes. We know that in order to have a connecting rod equally strong about both the axes, Ixx = 4 Iyy
where Ixx = Moment of inertia of the section about X-axis,  and Iyy = Moment of inertia of the section about Y-axis.
 In actual practice, Ixx is kept slightly less than 4 Iyy. It is usually taken between 3 and 3.5 and the connecting rod is designed for buckling about X-axis.
Now, for the section as shown in Fig.  area of the section,
Now let us find the dimensions of this I-section. Since the connecting rod is designed by taking the force on the connecting rod (FC) equal to the maximum force on the piston (FL) due to gas pressure, therefore,   
                                                                                                                             
We know that the connecting rod is designed for buckling about X-axis (i.e. in the plane of
motion of the connecting rod) assuming both ends hinged. Since a factor of safety is given as 6, therefore the buckling load,
WB = FC × F. S. = 24 740 × 6 = 148 440 N
We know that radius of gyration of the section about X-axis,
Length of crank, r = Stroke of piston/2 = 190/2= 95 mm
Length of the connecting rod, l = 380 mm ...(Given)
Equivalent length of the connecting rod for both ends hinged,
L = l = 380 mm
Now according to Rankine’s formula, we know that buckling load (WB),
                                                                             
Thus, the dimensions of I-section of the connecting rod are :
Thickness of flange and web of the section 
= t = 7 mm Width of the section, B = 4 t = 4 × 7 = 28 mm
and depth or height of the section,
H = 5 t = 5 × 7 = 35 mm
These dimensions are at the middle of the connecting rod. The width (B) is kept constant throughout  the length of the rod, but the depth (H) varies. The depth near the big end or crank end is kept as 1.1H to 1.25H and the depth near the small end or piston end is kept as 0.75H to 0.9H. Let us take 
Depth near the big end, H1 = 1.2H = 1.2 × 35 = 42 mm and depth near the small end,
H2 = 0.85H = 0.85 × 35 = 29.75 say 30 mm 
Dimensions of the section near the big end = 42 mm × 28 mm .
and dimensions of the section near the small end = 30 mm × 28 mm
Since the connecting rod is manufactured by forging, therefore the sharp corners of I-section
are rounded off, as shown in Fig., for easy removal of the section from the dies.
2. Dimensions of the crankpin or the big end bearing and piston pin or small end bearing
Let dc = Diameter of the crankpin or big end bearing,
lc = length of the crankpin or big end bearing = 1.3 dc ...(Given)
pbc = Bearing pressure = 10 N/mm2 ...(Given)
We know that load on the crankpin or big end bearing = Projected area × Bearing pressure
= dc .lc . pbc = dc × 1.3 dc × 10 = 13 (dc)2
Since the crankpin or the big end bearing is designed for the maximum gas force (FL), therefore,
equating the load on the crankpin or big end bearing to the maximum gas force,
 i.e. 13 (dc)2 = FL = 24 740 N
∴ (dc )2 = 24 740 / 13 = 1903 or dc = 43.6 say 44 mm Ans.
and lc = 1.3 dc = 1.3 × 44 = 57.2 say 58 mm Ans.
The big end has removable precision bearing shells of brass or bronze or steel with a thin lining
(1mm or less) of bearing metal such as babbit.
Again, let dp = Diameter of the piston pin or small end bearing,
lp = Length of the piston pin or small end bearing = 2dp ...(Given)
pbp = Bearing pressure = 15 N/mm2 ..(Given)
We know that the load on the piston pin or small end bearing = Project area × Bearing pressure
= dp . lp . pbp = dp × 2 dp × 15 = 30 (dp)2
Since the piston pin or the small end bearing is designed for the maximum gas force (FL),
therefore, equating the load on the piston pin or the small end bearing to the maximum gas force,
i.e. 30 (dp)2 = 24 740 N
∴ (dp)2 = 24 740 / 30 = 825 or dp = 28.7 say 29 mm and lp = 2 


dp = 2 × 29 = 58 mm

The small end bearing is usually a phosphor bronze bush of about 3 mm thickness.
3. Size of bolts for securing the big end cap                                               
Let dcb = Core diameter of the bolts,
σt = Allowable tensile stress for the material of the bolts
= 60 N/mm2 ...(Given)
and nb = Number of bolts. Generally two bolts are used.
We know that force on the bolts

The bolts and the big end cap are subjected to tensile force which corresponds to the inertia
force of the reciprocating parts at the top dead centre on the exhaust stroke. We know that inertia force of the reciprocating parts,

We also know that at top dead centre on the exhaust stroke, θ = 0.
Equating the inertia force to the force on the bolts, we have

and nominal diameter of the bolt,

4. Thickness of the big end cap
Let tc = Thickness of the big end cap, bc = Width of the big end cap. It is taken equal to the length of the crankpin or big end bearing (lc) = 58 mm (calculated above)
σb = Allowable bending stress for the material of the cap = 80 N/mm2 ...(Given)
The big end cap is designed as a beam freely supported at the cap bolt centres and loaded by the inertia force at the top dead centre on the exhaust stroke (i.e. FI when θ = 0). Since the load is assumed to act in between the uniformly distributed load and the centrally concentrated load, therefore, maximum bending moment is taken as
                                            
                                            = Dia. of crank pin or big end bearing + 2 × Thickness of bearing  
liner + Nominal dia. of bolt + Clearance = (dc + 2 × 3 + db + 3) mm = 44 + 6 + 12 + 3 = 65 mm

     
   Detailed drawing.

Manufacturing Process of Connecting rod.
Conclusion
    The project on the design of connecting rod with respect to the Industrial engineering    gave us platform to understand the design process of connecting rod in detailed , also scope for further studies on advance research on connecting rod and usage of IE tools for the manufacturing process like JIT.
    References
    1. Machine Design book By R.S.Khurmi.
    2. www.google.com
    3. www.youtube.com