Mechanising the Dome Shutter

As described earlier, the dome was built back in 1983 and for 16 years we opened the shutter, an up and over type, by lifting it manually. This article describes the mechanisation of it.

The shutter opening is some 3’ wide and 12’6” long. This length, plus 6” beyond the top and 6” below the bottom, was divided into 6 sections. 6 sections, because that was the least I could get into the open length with straight sections. Straight sections, because the mechanisation involves bicycle chain over pulleys and I didn't want chain hanging down below the ribs either side of the shutter opening. Bicycle chain, because it's easily obtainable. We actually got members to go into their local cycle repair shop and ask them to retain all the discarded used chain for us. New chain would have been too costly. I then washed them all in white spirits and chose those that were least worn. The total length needed was about 16M and we needed more than twice that to get enough good chain. Not only did the chain all have to be of the same type but as it was all used chain it all had to be worn to the same degree. This, to ensure both chain lengths, one each side of the shutter opening, would be the same length between top and bottom sprockets.

There is a pair of pulleys between each ‘straight section’ of chain, one outboard of the other. Each chain is connected to the shutter itself at the bottom end. It runs in a long loop, up over the top of the outboard pulleys, around the sprocket beyond the top end of the opening, and back again along the top of the inboard pulleys. Then down and around the sprocket below the bottom end of the opening, back to the same fixing point on the bottom end of the shutter.

 A 1" Dia. steel shaft carries the sprockets at the top and another carries the bottom sprockets. The sprockets are locked to the shafts (no free-wheels) and the shafts run in nylon bushes in steel housings within the thickness of the ribs. The shafts with the sprockets locked to them keep the chains running together and ensure the drive load is the same both sides of the shutter.

The drive is by handle through chain reduction gearing to the bottom shaft. The pulleys (20 in all) are turned from nylon bar and are 2" Dia by 5/8" wide with a 13/32" Dia bore and a groove to carry the chain. They run on cantilevered shafts secured to the sides of the shutter opening. The shafts are standard 10mm bolts about 85mm long that have had their thread length increased to just 5/8" short of the head.

The pulley and shaft assembly consists of the bolt, the pulley, two thrust washers, two locknuts and a washer and locking nut. The pulley is fitted to its bolt together with the thrust washers and plenty of grease and retained in position, with sufficient side clearance, by the lock nuts. The bolt is pushed through a 10mm hole drilled through the rib and retained there with the washer and locking nut on the other side of the rib. The pair of locknuts sink into a prepared counter bore in the rib face. When tightened up, the counter bore is deep enough to ensure the pulley runs freely just clear of the side face of the wooden rib.

The sprockets are all bicycle sprockets of course, and fasten to their normal bicycle hubs by means of three keys in the sprocket bore engaging with three keyways on their hubs. We had therefore to produce hubs of a similar design. We needed six. Two for each cross shaft and two more for the drivebox. All bearings, for both cross shafts, and the short drive shaft, are nylon bushes in steel housings. We were fortunate in having a new member, Keith Wood, who had a lathe and a mill who could do all this work for us. It was almost as though we were waiting for him to join.

The drivebox consists of a short shaft carrying a handle on a 6" long arm and a 17 tooth sprocket, the same size as those on the cross shafts, making five in all. This drives a 32 tooth sprocket on the end of the bottom cross shaft by means of a short loop of chain. The drivebox itself is a weldment in 3mm sheet steel, bolted onto the right side of the right rib at hand height. The gearing provides for a load of about 12lb max. at the handle, and 29 turns to raise or lower the shutter. Security is a bolt through the handle’s arm into the weldment.

There is provision for adjusting the length of the chains at the fixing points to ensure they are the same length and to cater for wear. To fit the system on the dome, holes had to be made for the chains in the wooden step at the bottom of the shutter opening and through the wooden top brace that holds the two main ribs together. To ensure the dome structural strength wasn't compromised, a steel brace was fitted at the top to carry the loads the now weakened top brace couldn’t.

Faced with the problem of members getting oil on their clothes from the chains if they leaned on the sides of the shutter opening, I’d planned to house the chains in sheet steel covers. But in the event, decided to oil the chains, wipe them clean, then paint them. And that seems to work well, as does the whole system.

Brian Joynes