Machine Designers Reference. J. Marrs
be the skill level of the operator?
•Is repetitive strain injury a risk for this device or tool?
•Is failure of this part or device a safety hazard?
Environmental Considerations:
•What forces will the device be exposed to?
•Will the device be exposed to extremes of temperature?
•Will the device be exposed to fluids, dirt, or other contaminants?
•Will the device be exposed to gases other than air?
•Must the device be food safe, wash-down, FDA compliant, or meet other cleanliness standards?
•What sources of power (human, electrical, fluid, etc.) are available to run the device?
•What are the maintenance capabilities where the device is installed?
•How much space can the device occupy?
•What is the carbon footprint of the device?
When writing a specification for a machine or device that is to be built by a supplier, it is doubly important to get every known or expected detail about the device in writing to guide the design and avoid misunderstandings. In the case of complex machinery, a functional specification can be 50 to 100 pages long. Every organization typically has its own preferred format and contents.
The following is a simplified example of a functional design specification for a fictional shrink wrap machine. In this example, the possibility of using a heat tunnel has already been eliminated, so it is part of the specification that a heater must be moved into position temporarily to heat the shrink wrap. If a heat tunnel was still a possibility, the functional part of the specification would be much more vague to allow for either possibility.
MACHINE: 161B Shrinkwrap Machine
INPUT: Product 161B, enclosed in shrinkwrap material (25 lb weight) presented in trays of 2
OUTPUT: Fully shrinkwrapped product 161B placed in trays of 2
CYCLE TIME: 30 sec. (2 parts per minute)
REQUIRED UPTIME: 95%
LIFESPAN: 10 years
# OPERATORS: 0.33 (1 per 3 machines)
OPERATOR TYPE: trained, English or German speaking, good reading skills
MAX OVERALL SIZE: 8 feet square, up to 15 feet tall
STANDARDS COMPLIANCE: EU standards, US standards
# DEVICES TO BE MADE: 6 in United States, 6 in Germany
UNITS: metric
INSTALLATION TYPE: Industrial environment, permanent, lagged to floor, no weight restrictions
FACILITIES: 480VAC, 3-Phase, Compressed Clean Dry Air 90psi
MACHINE FUNCTIONS:
1.Product is taken from tray and placed on conveyor in specific orientation.
2.Product moves into position under heater.
3.Heater lowers to product.
4.Heater dwells to shrink the wrapper.
5.Heater raises to safe position.
6.Product moves to cooling position.
7.Product dwells until cool.
8.Product moves to exit position.
9.Product is placed on finished goods tray in specific orientation.
FUNCTIONAL COMPONENTS:
•ENTRY DEVICE
°Takes product from tray in specific orientation
°Moves product to shrinkwrap machine entry position while maintaining orientation
°Must be automatic (not manual feeding)
•PART CONVEYANCE
°Moves product from entry position to heat position, cool position, and exit position while maintaining orientation of product
°Contact with product must be limited to areas specified on print.
°No rubbing or sliding contact with product is allowed.
•HEATER
°Heater must be 10mm from product and maintains 300°C temperature, according to experimentation.
°Heater must be in position 15 seconds to shrink the wrapper, according to experimentation.
°Heater must cover 50mm-diameter flat area and 20mm down the sides of the product in order to effectively shrink the wrapper, according to experimentation.
•COOLING
°Must be accomplished with ambient air conditions — no air jets or chillers are allowed in this application
°Product must cool from 300°C to 100°C before being placed in tray.
°Cooling to 100°C takes 25 seconds after 300°C heater removed, according to experimentation.
•EXIT DEVICE
°Takes product from machine exit position in specific orientation
°Moves product from exit position to tray while maintaining orientation
°Must be automatic (not manual removal)
INTELLIGENCE:
•Heater temperature control
•Machine not ready until heater temperature setpoint is reached
•Interlocking guard door for safe operator access
•Heater position error shuts down machine
•Conveyor jam sensing
•Operator panel must have machine performance data.
SAFETY REQUIREMENTS:
•Standards: US, EU
•Company requirements
°Fixed or interlocking guards around perimeter
°Roof on guard enclosure
°Noise level limit at operator position 80dB
°Warning label attached to thermal hazard
°Light tower 9 feet off the ground featuring green, yellow, and red at the top
SPECIAL REQUIREMENTS:
•Must allow conversion to run product 162B with 1-hour changeover time
•Operator must be able to access heater chamber from front AND back of machine.
The functional design specification is an organized list of what is known about the machine at this early stage. It should contain all the known constraints and expectations, but should be written in a way that avoids placing new constraints. For example, the product conveyance subsystem is not called an indexing conveyor here because it has not yet been decided what form the conveyance should take. An indexing conveyor is most likely, but at this point in the design process it is still open for consideration. Continuous conveyors, or pallet systems, are also possible conveyance solutions.
From this specification, the designer can begin to develop a timing diagram for the machine and determine how much time is allowed for each function. That is often the next step toward conceptual design of automatic machinery. It is important to note that the design specification details what the device must do, but not how it must be done. The question of how will be addressed in the conceptual and detail design stages. Concepts for the conveyance system and the individual devices that perform various machine operations (such as lowering and raising the heater) can then be developed and evaluated based on their ability to meet the functional design specification.
RESEARCH
The importance