Tuesday, 18 March 2014

Nut & Bolt - Big Time!

Years ago, my mother recounted the story of her first driving lesson in which, once she had got the car moving, her main concern was how on earth to stop it!

Health and safety aside, the principle has stood the test of time. Sentinel 7109's original handbrake screw thread was clearly beyond use when it first arrived at Midsomer Norton. I never took a photo of it but the square shaped 'Acme' thread bore a strong resemblance to the 60 degree angles of a metric thread profile! As the last resort for stopping, it did not provide much in the way of confidence!

The way the handbrake works is quite simple. It's a big vertical bolt with a handle and a nut that's linked to a crank. When the handle is turned, it pulls on the crank which pulls on the brake. (There's some more about it here: Steam Brake Cylinder).
Handbrake Nut and Linkage (below cab floor)
Handbrake Handle (above cab floor)
Best efforts were tried to repair the damaged thread by machining it away and welding new metal in place to allow a new thread to be cut.

It worked initially as in the video below (Also on YouTube).

However, after a while, the cutting tool decided it wanted to dig a little deeper and pulled off some of the metal causing a complete write-off of the original shaft.

Mendip Steam Restorations were tasked with the job and completed both the finishing of a new nut (with posh phosphor bronze 'Top-Hat' insert) and a new handle shaft.
New Handbrake Shaft and Nut
The original handle assembly has been attached to the top of the new shaft. I wasn't sure about a plain thrust bearing and decided that, for minimal cost, a roller thrust bearing could be used as shown below. Whether it will prove strong enough in practice, I don't know but at least there is the fall-back to the original construction.
Roller Thrust bearing below the handle
New nut & insert from below
New nut & insert from above
So, with any luck, we'll now be able to stop 7109 after we've made it go!

One feature which I do intend to add is a shield of some sort to prevent grit getting into the thread; hopefully this will prevent the damage that happened to the old one.
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