Thursday, 24 November 2011

Brazed Nuts and Gear Cases

After many years of staring wonderously up into the portals under Sentinel 7109's lower gear cases, 23rd November 2011 finally saw them closed up and an oil bath created inside.
Under-frame view showing the nice pair of closed-up lower gear case portals
There are a number of requirements to be satisfied with these cover plates: they must not leak around the gasket; they must not leak around the screw threads (the holes are open inside to the oil bath); they must allow condensate to be drained off from below the oil (all of it if possible).

The gaskets are hand made from a rubberised cork material about 1/8th inch thick (3mm) using the two plates themselves as the respective templates.

Plate and cork gasket (two of each)
Preventing leakage down the screw thread is not so easy. At first, studs were tried but they were loose in the thread. These could have been secured with Loctite of an appropriate grade; however, I was concerned that the seal might not be maintained for the many years it will be required. Also, if either plate needed to be removed, undoing the nuts on the studs could loosen a stud and require it to be refitted and resealed. So I chose to use 5/16" BSW hex headed bolts with fibre washers.

[Note to me: 6 of the 24 bolts have been cut to 1/2" length for the holes which have been closed off with weld where presumably there was a problem with the gear case casting. The rest of the bolts are 1" but could be cut to 1/2" in future if wanted - unnecessary extra work!].

I'd also been recommended to use 'Dowty' washers instead of fibre; however, I had to dismiss this idea as some of the holes in the plates were not round and would allow leakage at the long end of the holes! (Offers for 25 M8 Dowty washers anyone?).

I devised a cheap method for draining the condensate using M12 nuts brazed to the underside of the plates and drilled and tapped so that a bolt could protrude right through. I milled a slot along the length of the bolts so that they could seal when tightened up but let fluid pass when loosened.

Slotted bolt and brazed nut
The key point here is that fluid needs to drain down to the surface of the plate and not allow a pool of condensate to remain that cannot be drained. One suggestion had been to use a drain cock; however, it would still need a brazed nut or threaded boss to mount it and £15 or so each is a lot more than the cost of stainless steel M12 bolts!

Use Crankcase Oil
Sentinel require Crankcase oil to be used to lubricate the gears as there can be seepage of crankcase oil through the crankshaft end bearings from the crankcase to the gear case. After some careful investigation, I selected Hallett Oils' SCC680 Crankcase oil which is designed specifically for Sentinel steam engines. It is a viscous, gloppy (onomatopoeic word) fluid that encourages any condensate to separate out and sink to the bottom (water being heavier than oil). It's this property that enables the condensate to be drained.

The gear cases needed to be filled to the second check level to suit the diameter of the gear wheel inside.

The one in the middle
At this point I realised that the gear cases were somewhat larger than I'd anticipated as they swallowed at least two gallons per side! So the plentiful stock of crankcase oil purchased for the engines is not now as plentiful as it needs to be!
Right hand Cover Plate
Left hand Cover Plate
So let's hope that I've thought of everything to prevent any leaks as draining out two gallons of gloppy oil to start again is not an appealing prospect!

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