Lifting and lowering the main hatch by chain drive meant that the chain itself would need to be firmly anchored to the hatch at some point. Owing to the fact that it would need to be opened fully, the anchoring point had to be at the foremost edge of the hatch on the right which was the side chosen for the motor housing. The anchoring bracket itself was designed to join up with the links of the chain by a rivet and to the front corner of the hatch frame on the right by two short stainless steel screws.
The distribution and layout of the drive chain and the array of associated roller bearings, take up spools and spring tensioners were almost entirely arrived at by that age-honoured method of trial-and-error. This naturally involved several modifications to the initial design and ultimately took the shape of the diagram below that shows the two sprocket wheels, a large take-up one at the top and a driven one at the bottom of the loop. The intermediate wheels are six home-made “chain guides” made from stainless steel washers sandwiched over a roller bearing mounted over 8mm roofing bolts driven into the arched beam at the spots chosen. This allowed the chain to track easily and diminished the risk of derailment had I opted for sprocket wheels instead. It also allowed the anchoring bracket to glide over them without jumping over cogs and wearing them down over time.
To reduce chain sag with reversal of direction and especially at the ends of its travel the spring loaded guides seen in the diagram were added. The double one in the middle helped to keep the chain aligned and the one below, just above the driven sprocket was essential to keep the chain well apposed to the drive, taking up slack and compensating for the varying loads on the motor.
The anchor point in the diagram shows its location with the hatch fully closed.
This worked fine, lifting and lowering the hatch on reversal of polarity to the DC drive motor below BUT as previously mentioned the load on the motor varied with the position of the hatch and its direction of travel. Past the meridian in both directions, gravity would tend to speed up its descent and some form of braking was required. This needed some thought. Indeed I could not find anywhere any mention of this problem and any useful reference, so I had to devise my own solution.
Since a gradual brake was necessary on the descent in both directions past the meridian but was obviously not useful when the motor was lifting the load, some sort of toggle system was required that automatically and continuously adjusted the load on the motor drive.
… a sporting solution
I converted three of my home made “chain guides” to double ganged ones by adding another washer / roller-bearing gang on the inside of the ones carrying the chain, holding the two resulting channels tightly together by a locknut at the back. I added another of these guides to the back of the roofing bolt “axle” of the larger sprocket wheel at the top. I then drilled a hole in the horizontal support of the steel frame of the hatch on the right side about a third of the way from its lowermost edge and directly above and in line with the newly added roller guides.
A length of non-stretch polyester marine-type cord was fixed to the hole in the hatch frame and guided over the bearings as shown in the speeded up animation below.
The free end of the cord was then attached to one of the springs “borrowed” from a chest expander. This spring was channelled through a length of black garden hose pipe of the sort used for drip irrigation and the free end of the spring anchored to the rising right main arch beam at the back via a hooked tension screw. The garden hose was kept neatly tucked away in the corner on the top edge of the arched beam.
The length of the cord was cut to size to allow the spring to be fully relaxed when the hatch is at the meridian. The tension screw at the anchor point allowed fine adjustments to the spring tension to be made. When properly ”tuned” the spring would just peep over the top of the black tube at both ends of travel of the hatch. Silicone grease was liberally applied to all roller guide channels and silicone spray supplemented from time to time to reduce friction and wear on the chain.
The completed system worked well and the stored energy in the spring at the extreme ends of travel of the hatch in fact served also as a boost to the motor when overcoming the inertia at the start of traction.
Watch some of the stages in the making of this drive on youtube