Work Measurement - 1

文章目錄

• Work Measurement - 1
• • Overview
  • Introduction to work measurement
  • • Time is important
    • Time standard: definition
    • Standard time: Application and functions
    • Methods to determine time standards
    • Prerequisites for valid time standards
  • standard input work unit
  • average worker
  • standard performance
  • standard method
  • • Average worker
    • standard performance
    • performance rating
    • other things to standardise
    • how do we cope with rest breaks?
    • Allowances
    • Fixed allowances (simplest version)
    • Variable allowances
    • examples
    • other allowaces
    • summary of standard time
    • example
    • summary
  • direct time study
  • • definition
    • 7 steps
    • reasons for breaking operation into elements
    • timing techniques
    • time study template
    • Example
    • Measureement problems with direct time study
    • precision in work measurement
    • confidence
    • probability density function of measured time
    • confidence
    • probability density function of measured time
    • tabulation of student - t distribution
    • examples
    • number of cycles to be timed?
    • example
    • summary
  • learning curves
  • • learning curve phenomenon
    • learning curve
    • Learning curve theory
    • Typical learning rates
    • source of learning curves
    • Learning barriers: factors impeding learning
    • Interruptions to the learning curve
    • Summary
    • Next week

Overview

• work study

  • method study
    • method analysis
      • observation
      • charting
      • questioning
    • methods design
      • motion economy
      • ergonomics
      • Learn
  • work measurement
    • time study
      • CORE!
    • pmts
      • Pre-determined measurement time system
    • standard data systems
      • Data in standard system
    • work sampling
      • sample work for the operator’s domain is great

• Agenda

  • Introduction to work measurement
  • direct time study
  • learning curves

Introduction to work measurement

Time is important

• labour content
  • cost of labour time
  • major factor in the total cost of a product or service
• how much time will be required to accomplish a given amount of time

Time standard: definition

• amount of time
• allowed for an average worker to process one work unit
• using the standard method and working it at a normal peace

Standard time: Application and functions

• used to
  • fair day’s work
  • means to convert workload into staffing and equipment needs
  • Objective comparisons
  • basis for wage incentives and evaluation
  • time data for
    • production planning and scheduling
    • cost estimating
    • material requirements planning
• beneficial
  • low productivity
  • long production runs
  • Repeat orders
  • short and repetitive work cycles

Methods to determine time standards

• methods to set time standards
  • estimation (then to estimated, if under time pressure )
    • estimated times
      • Basis
  • historical records (firstly to consider)
    • historical times
      • pros
        • more accurate
      • cons
        • not adapted to changes
  • work measrement techniques
    • engineered standards
      • direct time study
      • predetermined motion time systems
      • standard data systems
    • work sampling
      • proportions of work activities

Prerequisites for valid time standards

• factors that must be standardized before a time standard can be set
• task (standard time for task)
  • input

    • standard input work unit

    • average worker

    • standard performance

    • standard method

  • Output
    • standard output work unit

Average worker

• worker
  • usually perform tasks
    • if male, then male
    • if female, then famle

standard performance

• pace of working
• maintained by an average worker
• distribution normal performance
  • 68%
• Engineering is not only about technology but also around the people
  • strength
  • activities
  • pins in the phones
  • Abilities

performance rating

• analyst judges the performance of pace of the worker relative to the definition of standard performance used by the organization

• standard performance rate PR = 100 %

  • slower pace than standard PR < 1
  • faster pace than standard PR >100%

• basic time

  • Tb=Tobs*PR

• example

  • analyst
  • Worker carrying out a task
  • worker does the job in 5 mins
  • 80% of the standard performance
  • basic/ normal time to do the job at 5 mins *0.8= 4 mins
  • Therefore, a standard worker time should be able

other things to standardise

• standard method
  • optomum method
    • safest, quickest, most productive and least stressful
• standard method
  • explains all actions and motions
  • include all of the details
  • On
  • how the task is performed
    • including
      • procedure
      • tools
      • Equipment
• standard work units

how do we cope with rest breaks?

• two approaches

  • Scheduled rest breaks during the shift
    • typical one 15 min break in mid morning and another in mid-afternoon
    • mainly used where worker interaction is essential
  • PFD (personal time, fatigue, distraction) allowance

• not the same time

• Time

  • Lunch time does count towards breaks!!!
  • protect their people in their lunch
  • always protect your people for the lunch!!!

Allowances

• basic time adjusted by allowance factors to obtain the standard time
• purpose
  • account for fatique
  • Recognize that workers will not spoend the whole shift working and cope with other contingencies
• total allowance
  • relaxation allowance
    • fixed allowances
    • variable allowances
  • contingency allowances

Fixed allowances (simplest version)

• personal times
  • toilet breaks
  • getting drinks
  • 6% typical
• basic fatigue
  • Rest allowance to overcome general fatigue (4% typical)

Variable allowances

• given only when the working conditions vary from those which are assumed in the basic allowances
• physical strains
  • force
  • posture
  • vibration
  • short cycle
  • restrictive clothing
• Mental strins
  • Concentration/anxiety monotony
  • eye strain
  • Noise
• nature of working conditions
  • Temperature and humidity
  • dust, dirt
  • Wet

examples

• sawubf wood
• outdoors
• dry
• autumn
• 15 degree
• sawing force
• wear
• physical
  • force
  • posture
  • vibration
  • short cycle
  • restrictive gloves
• mental
  • concentration
  • monotony
• working conditions
  • Guest 2
• total allowance points: 54
  • 26 %

other allowaces

• contingency
  • additional allowance due to a problem with the break
• training allowance
• learning allowance
• Policy allowance

summary of standard time

• basic time
  • observed time *
• standard time

example

• packing boxs
• average observed time for a repetitive work cycle is 3.27 mins
• work performance was rated by an analyst at 90%
• allowance factor is 0.13
• what is the standard time for this task
• observed time
  • 3.27
• basic time
  • 3.27*0.9=2.94
• standard time
  • 2.94*1.13= 3.33 mins
• 3.33 mins per box

summary

• time standard
  • average worker
  • standard performance

direct time study

definition

• times and rates of working

7 steps

• basic information about the operations
• define and document the standard method
• divide the task into work elements
• time the work elements and calculate the mean value to obtain the observed time
• Evaluate worker’s pace relative to standard performance
• apply the allowance factors?

reasons for breaking operation into elements

• analysis the operation by elements may show slight variations
  • could not be detected so easily from an overall study
• an operator may not work at the same tempo throughout the cycle
  • An elemental time study permits seperate performance ratings
• different elements may require different timing

timing techniques

• snapback time
  • Watch is started at the beginning of every work element by snapping it back to zero at the end of the previous element
  • the analyst must note and record the final time for the element just as the watch is being zeroed
  • advantages
    • see how the element times vary from one cycle to the next
    • no subtraction is necessary to obtain individual elements
• continuous timing
  • watch is zeroed at the beginning of the first cycle and allowed to run continuously throughout the duration of the study

time study template

• feed
  • Timing
• speed
  • how many inputs are they using

Example

Measureement problems with direct time study

• how can we be sure that the limited sample of times observed is actually representative of the truth?
  • is the mean of the observed times equal to the true value of the time to perform the task?
  • unless you make a very large number of observations, you can not make that claim
• however the number of observations that can be made during a direct time study is liited
  • cost
  • time
  • effort
• how many observations must be made so that we are fairly confident about the outcome of the study?

precision in work measurement

• precision is concerned with the
  • expected variability within a single time study
• precision of a time standard is determiend a certain confidence level
  • standard time
  • 4 mins
  • 98%

confidence

• to cope with variations

• time a number of repetitions of the job

• confidence interval for the mean value of the time

• assume observed work element times

• normally distributed

• observed means

• observed data

• sample size

probability density function of measured time

• n: sample size
• 95% confidence
• 10% from the observed mean
• Probability density
• interval size is at 10% of the x bar
• (1-\alpha)
• Sample size is usually small
• standard normal distribution
• t-distributed with n-1 degrees of freedom

confidence

• actual standard deviation sufna
• sample standard deviation
• sample size is small

probability density function of measured time

• alpha = 0.05

tabulation of student - t distribution

• degrees of freedom n-1=29
• probability of 95%

examples

• 9 cycles of assembly operation have been observed during a direct time study
• Mean time is 1.3 mins
• the corresponding sample standard deviation
• a = 0.05
• two tails
• 1-a/2-0.975
• t_{n-1}^{a/2}=2.306
• 95% CI

number of cycles to be timed?

• how many observations we need to take
• k = 10%
• max fractional error
• we are prepared to accept
• number of observations to take
• The practicality/ cost vs accuracy trade off

example

• time study analyst

• 10 readings on the particular work element of interest

• consider how many cycles to time

• based on the sample

• Mean tine for the element is 0.4 min

• sample standard deviation is 0.07 min

• 95% confidence level

• how many cycles should be timed to ensure the actual element time is within ±10% of the mean?

• Ans 6 answer

• $$n=(\frac{t_{n?1}^{1?\frac{\alpha }{2}}s}{}{)}^2$$

summary

• direct time study
• precision in work measurement
  • confidence
  • probability density function of measured time
• 0.95 is the standard rate that everybody is working

learning curves

• the more you do

learning curve phenomenon

• reduction in cycle time
• as the number of cycles increases
• when a worker accomplishes a task over and over

learning curve

• X-axis

  • Units produced

• Y-axis

  • Processing time per unit

  • y
    $$y=\frac{time}{unit}$$

Learning curve theory

• learning rate
  • Example

Typical learning rates

• type of work

  • assembly
  • prototype
  • clerical ops
  • inspection
  • machining
  • Welding

• LR

  • 85
  • 65
  • 75-85
  • 85
  • 90
  • 85-90

• the higher the customized domain

  • the higher the learning rate is

• list of learning rate

• higher learning rate

source of learning curves

• self
  • practice
    • Worker becomes familiar with the task
    • moves from conscious
    • Sub-conscious
  • Worker makes fewer mistakes
  • Hand and body motions more efficient
  • reduce distances
• organizations
  • fine tuning of machinery and tooling
  • development of special tooling
  • product design improvements
  • leadership and motivation of workers

Learning barriers: factors impeding learning

• physical and mental limits
• frustration
• inertia
• Interruption/forgetting
• Constraints imposed by other memners of group
• shortage of resources, lack of investment in new technology
• lack of leadership
• low motivation of workers
• Good quality
  • if not, more money and more time

Interruptions to the learning curve

• learning curve effect tends to be disrupted by interruptions in production
• for example due to
  • Batch production
  • between repeat orders
  • labor strikes
  • holidays
  • raw material shortages
• emissions
  • when production is resumed after a break

Summary

• calculate standard time
• effect of the learning curve

Next week

• Work measurement 2

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