Here is the finished system. The project was premised on the availability of close-assist springs - which turn out to be about five times the price of the 'regular' ones. So one redesign later...
The head weighs around 220lbs and the Z slide probably another 40 - 50lbs. The gas spring (from Mcmaster) has close to 10" of travel and a 225lb rating. However, the head travel is around 17", so I set the spring side up with an extra pulley to halve the travel, but at the expense of doubling the load; so the spring will actually see around 500lbs. In practice however, presumably because of the cumulative friction (the ways, chain and all the bearings) the system holds the head even when the stepper is not powered. I angled the chain away from the axis of the spring slightly with the intention of equalizing the radial loading. However, thinking about again, I'm not sure this was the best plan. The whole setup is adjustable so I may move the chain in further to reduce the angle or replace the single spring with a pair that are offset to generate stability in the weak axis. But if it ain't broke...
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