Tuesday, 27 June 2006

Case Study : Robot Assembly of Pneumatic Valves (1/3)

This is a quick feasibility study that I was asked to carry out for a Swedish world leading manufacturer of compressors, generators, construction and mining equipment, industrial tools and assembly systems.  They wanted a swift appraisal of the economics for the robot assembly of their pneumatic valves.


The client company has committed to capital investment in a two-armed robot for the assembly of its range of pneumatic cylinders at one of its Swedish manufacturing plants.  The robot will not be fully utilised and another product is required to economically justify the installation.

The manufacturing plant currently assembles two product families :

1) Pneumatic cylinders
2) Pneumatic valves

The feasibility of assembling a model of pneumatic valve is investigated for the client company.


The pneumatic valve annual production volume is 30 000 units.  The valve is currently assembled manually and the client company assumes that demand for the valve will increase to 40 000 within 2 years.  Three workers are currently required for product assembly.  The product has a total of 63 separate parts, of which, 31 are unique parts.


The manual assembly of the valve has been studied to create individual times for the 97 operations.  The manual assembly worksheet, shown below, gives the sequence of operations and their corresponding operation times.  The worksheet shows that the cycle time for the complete assembly is 389 seconds.  One worker can assemble 11 786 valves in one year, with single shift working at a labour efficiency of 70 percent :

225 working shifts per annum (single shift working)
= 5 940 000 working seconds per annum (440 minutes / shift)
= 4 158 000 working seconds per year at 70 percent labour efficiency
= 10 689 units assembled per annum.    .


Certain assembly operations can be executed by the robot without a re-design of the pneumatic valve.  The assembly sequence for the robot assembly is given at the end of this article and an estimate for the robot capital expenditure is :

(a) Cost of the robot and controller = 50 000 euros.

(b) Turret and eight grippers = 5 000 euros

(c) Fixture number 32 = 3 000 euros, fixture number 33 = 2 000 euros, fixture number 34 = 2 000 euros, fixture number 35 = 3000 euros, fixture number 36 = 2 000 euros, fixture number 37 = 500 euros, fixture number 38 = 500 euros, fixture number 39 = 2 000 euros

(d) Arm-2 0-ring tools (6 off), including tool holder = 1 800 euros.  Arm-2 screwdriver bit and friction screwdriver bit = 200 euros.

(e) Greasing station = 2 000 euros

( f) Labelling station = 5 000 euros

(g) Cleaning station = 2 000 euros

(h) Eight vibratory linear feeders at 3 000 euros each = 24 000 euros

Total = 105 000 euros


It is estimated that the cycle time for the robot and manual assembly of the valve would be 456 seconds.  Using this estimate, one robot can assemble 5210 valves in one year, with single shift working at a robot efficiency of 80 percent :

225 working shifts per annum (single shift)
= 5 940 000 working seconds per annum (440 minutes/shift)
= 4 752 000 working seconds per year at 80% robot efficiency
= 10 421 units assembled per year (single shift)

The robot can assemble approximately the same number of products per year as one worker, considering single shift working.  However, certain operations (using the existing product design) must be executed manually.


The annual cost saving of using the robot is one worker per year.  If the annual cost of an operator is 50 000 euros per year (including taxes, social charges, pension contributions, overhead contribution, etc.) then the cost saving would be approximately 50 000 euros per year.


The payback period for using the robot is 2 years, for the assembly of 10 421 units per year.


If the valve is to be re-designed then it must have the following performance characteristics :

(a) It must achieve a flow rate of 2.2 litres per second for 10 000 000 cycles of operation, without leakage from port to port.

(b) The upper sealing gasket must not drop off when the body sub-assembly is transported between operations.

(c) The inner sleeve 0-rings must be stable during assembly of the inner sleeve sub-assembly to the valve body.

(d) The activation time of the unit must be better than 0.02 seconds.

(e) The operating air pressure for the double acting valve should be lessthan 1.2 kg/cm2 and less than 2.5 kg/cm2 for the spring return valve.

(f) The customer should have the option of achieving flow rates between 0 and 50 percent of the maximum and between 50 and 100 percent of the maximum, using a convenient design feature.

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