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Featured Engineering Project:

Reverser Screw & Nut reconditioning:

Today we focus on reconditioning the nut and lead screw for the locomotive reverser mechanism. For some time the nut and cast iron slider has been showing signs of excessive wear to the bearing surfaces, further investigation also presented uneven wear along the lead screw flanks causing the sliding nut to rock as the reverser is operated. Our intention is to re-cut the original lead screw thread (3 start ACME left hand), machine the slider faces and manufacture a new nut using EN16R to suit.

To begin with we first completely strip and clean the assembly taking note that the nut is off-set to one side by some 0.200". Because we intend to recondition the existing screw we must start the process in the reverse order to normal (in that one would usually manufacture a nut before a bolt because of the ease to alter a bolt compared to a nut), so we first mount the lead screw into our centre lathe and clock to true. We determine that the thread is of ACME style but at 15 degrees instead of the normal 14.5 degrees - the thread is a course 3 start 1.5" pitch left hand which means that for our Dean Smith and Grace lathe we need to change the screw-cutting back gears as well as using the 10X divider gear at the head stock. Because of the helix of the thread pitch we have to grind up a special tool to ensure that we don't run into tool foul problems whilst chasing the thread out.

Now we have the lathe set, starting at the headstock end of the job we cut each of the three starts being careful to index the compound slide exactly 0.5" between each of the threads (1.5" pitch divide by 3 starts = 0.5" index distance per thread)

We re-cut each surface of the threads taking the minimum amount possible from the material to obtain a good clean finish to the faces. Luckily the wear was fairly constant about the centre of the shaft with only a few gouges nearing the ends which should polish out quite easily. We take a small skim from the outside diameter as well as the core of the thread to ensure consistency (which makes producing the nut a lot easier if all three starts are exactly the same dimensions ie. all wear has been removed)

The next part of the job is to machine the cast iron nut slider on the reverser bracket. The casting is carefully set onto the horizontal borer so the the slider face is precisely vertical and perpendicular to the borer head face. I use the newly machined lead screw as a clock guide to ensure that the bushings (which are in good condition) are parallel to the sliding face that is to be machined. A boring bar is used in the chuck head which is offset so that I can cut the entire slider face in one vertical pass. A lot of care is taken to ensure that all faces are central and square with the shaft bushings.

Once that the machining of the shaft and the slider are complete we can turn our attention to the nut knowing that we have some dimensions to work towards. Because the nut is EN16R we must start from round stock (EN16 in certificated R condition is only available in round stock). First we saw the billet into a rough square profile, the billet is then passed to the shaper machine to be squared to a rough size. (the job is done complete by only one of our engineers, the shaper is used as it can run unmanned while the shaft is machined). To finish the roughing process of the profile the billet is moved from the shaper to the milling machine where I accurately centre drill and square the block to aid setting in the lathe.

The block is machined as near to perfectly square as possible in the mill so that if necessary I can manipulate the piece in the lathe four jaw chuck whilst knowing that as long as the material faces are parallel or square to the chuck face then the thread location should be easier to centre to the machine.

As this is an unusual thread, we first have to make a custom boring bar (EN24T) with a HSS insert (Swedish high cobalt HSS) so that we can ensure that we have enough clearance for the helix angle without fouling the cutting tool.

The 3 start left hand ACME is started by machining a very small recess in the face of the billet to the finished thread depth, this is is act as a machining aid as I expect a lot of boring bar bend during the machining process. Once the lathe change gears are checked for correct pitch I begin the long machining process.
As a general rule when machining ACME threads, the tip or insert is ground to match the helix angle of the thread (so it looks 'tipped over') and also thinner that the finished thread width. Roughing cuts are taken over all starts to within about 0.010" of finished depth. It should be noted that as with all screw-cutting the tool should be slightly indexed back after each cut to create a significantly smaller cut on the trailing edge of the tool rather than on the leading edge - this massively reduced chatter and grab. When machining very deep threads it may be necessary to complete machining by turning the job around in the chuck and machining a last cut from the other side (this gives a better flank finish on both edges of the thread.)
Eight hours later, and we try the male screw into the nut - I have given the thread some 0.010" - 0.015" thou clearance as I definitely don't want the steel on steel threads to bind if a bit of ash or grit were to work its way into the nut. Because of the off centre loading on the lathe I have also attached some counter balances to the chuck face to stop the lathe headstock gearing being driven unevenly as the weight of the job rotates - this is always good practise where possible to give the machine as easier time and also to help maintain a good even cut.

Now we're finished the turning process, the nut is transferred to the mill to have the sliding surface machined - very accurately positioned and finished so that the centre height of the thread related exactly to the reverser horizontal slider faces, an oil way is also cut to aid oil dispersion over the sliding faces.

We then pass onto the horizontal boring machine to insert the yoke pivot pin hole, this is some 0.002" - 0.005" thou clear of the hardened yoke pin as we need the mechanism to move freely within the nut block.

The nut is finally placed back into the mill to have an oil cavity/reservoir cut into the top face with vertical oil-ways providing lubrication to the thread, yoke pin and also the slide-way. A felt wick is placed into the oil-ways and gallery to allow the lubricant to slowly work its way onto all of the moving parts.

Two lube points are drilled into the reservoir cover before being screwed tightly to seal the top of the unit. We re-use the original position pointer which is also screwed and locked onto the top plate.

A final inspection and lube of the three start left hand ACME thread before we completely assemble the unit. We manufacture a new detent plunger for the locking mechanism as well as a new locking finger (both of which are hardened using EN24). The nut is run several times over the length of the screw to ensure that the mechanism moves easily, we also take note that oil is slowly being fed from the reservoir to the moving parts. All of the bearings are given a good greasing and the reverser is returned to the customer to continue to serve for many more years to come.
 
Component was featured on the website home page (February 2014).

 

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