Faster turnaround for robotic tooling
Original article date: November 1998
A tool failure in production can stop the production line which is very costly. Robotic tool changing provides flexibility to robot applications by allowing the robot to change end-effectors (grippers pneumatic and electric motors weld guns) automatically.
There are several robotic tool changers now available on the market differing principally in the locking mechanism which couples the master plate usually mounted to the robot arm to the tool plate which holds the tool to be exchanged.
The Quick Change tool changer originates from a company based in Garner North Carolina called Assurance Technologies. The UK distributor is TIA Tatem Industrial Automation. Its locking mechanism consists of a large pneumatically activated double-tapered piston high strength ball bearings and a heat-treated lock ring. The balls are retained in a holder near the piston on the master plate.
The Quick Change has a large moment capacity due to the locking piston’s large diameter and second taper. To lock the master plate to the tooling plate which is attached to the tool air enters the piston chamber through an inlet port in the master plate As the piston drops into the tooling plate moving from the unlock to lock position the ball bearings move outward from their resting position in the holder. The balls ride first on a 45deg angle on the nose of the piston (first taper) then along a flat surface on the piston and finally to a 15deg angled surface (second taper). Once in this position the balls are held in place between the piston in the master plate and the lock ring in the tool plate. This design accurately positions the master plate to the tooling plate.
The locking mechanism has a built-in and patented fail-safe feature that keeps the Tool Plate locked to the Master Plate should the locking pneumatic pressure be accidentally removed. The weight of the payload moves the piston slightly until the balls rest on the flat surface of the piston between the two tapers. In this position the balls are trapped and cannot move without air pressure being applied to the unlock port. This feature provides a reliable mechanical stop since the balls would need tob e crushed before the tool plat would disengage from the master plate (!) and it eliminates the need for a spring.
Should the taper surfaces experience any wear over time the piston compensates by allowing the balls to ride higher up the taper thus maintaining repeatability. Two high precision pins in the master plate are used to assure rotational repeatability.
How to Select a Robotic Tool Changer
Sizing (Fast Method)
If your moment is low or moderate select a Quick Change model with a payload rating similar to the robot it will be mounted on. If your moment is high move to the More Exact Method:
Sizing (More Exact Method)
Moment capacity ifs a critical factor in selecting the proper Quick Change model.
- Find the approximate centre of gravity of the heaviest end effector.
- Calculate the distance D from the centre of gravity to the bottom of the tool plate.
- Calculate the weight W of the heaviest end effector.
- Calculate WD to get an approximate static moment M
- Select a Quick Change with a moment capacity equal to or greater than M.
Robots may produce dynamic moments two to three times higher than M due to their potentially high acceleration. Quick Change models with a rated moment capacity of M are designed to handle dynamic moments that are three times higher than M.
Pneumatic and Electrical
Determine the number and size of pneumatic ports and electrical contacts needed. Larger Quick Change models have larger and more numerous pneumatic ports and electrical contacts.
Repeatability
Check the repeatability specifications when dealing with applications that require high repeatability.
- TIA Tatem Industrial Automation
- Steve Tatem
- 01332 204850
November 1998