Tapered Fits

[Rick Rowlands]

On my 10" rolling mill the mill couplings are attached to the cast iron rolls with a tapered fit. The Big end of the taper is a little over 4" dia., the taper is about 4.25" long and the small end is only about a tenth of an inch smaller in diameter. There are no setscrews, keys, or anything else holding the couplings in place. I can't figure out how it holds together.

One of the couplings has the tapered end of a roll broke off flush in it. I tried pressing that roll end out of the coupling with about 75 tons of force and it would not budge. I'm now whittling it out by drilling out as much metal as possible and hoping it finally frees up. The roll end is cast iron and the coupling is steel.

I would just like to know how common this type of tapered fit is, and how it holds so well.

 

[The real Leigh]

Hi Rick,

Taper fits are quite common, and they can hold quite securely, as you've discovered.

Shallow angle tapers (<10° or some such) are "self locking", in that once mated they tend to hold together even under significant stress.

This method is used with Morse tapers, Jacobs (chuck) tapers, and similar parts. Have you ever tried to disassemble a Jacobs chuck from its arbor? You can't do it.

Machine tool draw chucks, like R8, 5C, etc., have larger angles which do not lock when mated. These tapers serve only to apply pressure to the work or tool being held.

- Leigh

 

[moonlight machine]

A long slow taper like that gets so freeking tight it is amazing how well it holds. when it was assembled they may have heated the hub first, then press it on with 50 or 75 tons, it may as well be welded. I used to work on Aston Martin cars. to change the rear wheel bearing or seal the flange that holds the wheel needs to be removed from the axle. it also has a long slow taper, but it also has a key and a 7/8" or so nut to hold it on. to get it apart, the best way to do it was after it was set up in the press, have the big nut backed flush with the end of the axle. put a chain from one wheel stud around the press table to another stud. This is on a 50 ton Dake press by the way... Pump it up to 50 tons, then hit the table by the axle with a sledge hammer.....KAPOW!!! as it pops apart the press jumps about a foot off the floor, when the dust settles you are glad you chained the axle to the press. This taper is only about 1 3/8 dia., if it takes this much to get this little thing apart think what it will take to get a 4" dia hub off......

 

[Forrest Addy]

A tapered shaft/hub is a common feature of shipboard machinery in the Navy from li'l guy on small pumps to 22" ************ couplings in propellor shafts. I done lots of 'em.

Usually these tapered fits are set by the designer establishing a "draw" specification intended to result in a certain hoop stress in the hub. Let's asume the permissable hoop stress is 35,000 PSI (Yield strength for unheat-treated medium carbon steel).

You say the taper is 0.1" in 4 1/4" that's about 0.282" per ft. I assume your estimation of taper was just that. 1/4" / ft is a common shallow taper.

OK 4" dia x yield / young's modulus = 0.004." 35,000 PSI x projected area of fit (4 1/4" x 4" x Pi) = 930 tons (about) x 0.3 for coefficient of friction of steel Vs cast iron equals about 280 tons to break the friction. 80 tons is a fart in a whirlwind.

Assuming the taper is 1/4" per ft and it's properly fitted the draw would be (0.004" interferance / taper per inch = 0.192") a hair over 3/16".

The way to remove these taper fitted couplings is to put them under the available tension with a press or a puller then pour the heat to it with a BIG rosebud tip. A soft gas flame is not enough. It has to be an oxy-fuel flame if stationary is capable of heating a spot of solid metal the size of a quarter to red heat in one minute. The heat input to the hub has to be rapid. You have to expand the hub before the heat diffuses into the shaft. Keep the heat moving so the whole hub exterior heats and expands taking the cooler bore with it. Hesitate for a moment and you will hot spot the hub and possibly distort it. Sooner or later there will be a hell of a bang, the hub will jump, followed by an avalanche of puller hardware - mind your toes.

When you re-install the hub, machine the shaft taper so it blues in 80% and marks for the full length. If there is a key, fit it snug but not so tight the hub won't slip past it. Metal to metal almost. Deburr and clean. Heat the hub to 350 degrees F. Arrange a stop to prevent the hub from over-traveling and slip it on. Hold it in position against the stop and wait for it to cool. When cooled on a shallow taper the hub is as good as welded on but it does need some form of axial keeper just in case.

My figures are quick and dirty maybe they should be checked.

 

[Rick Rowlands]

Thanks for the info. It has broadened my understanding of taper fits. The mill was built by Danieli in Italy so the dimensions are most likely metric. Here is a shot of similar mill rolls showing the ends with and without couplings. There are plastic slippers inside the coupling so I hesitate to put heat to it until I determine a way to remove them. I can't get replacements.

 

[gbent]

On the tapered joints such as tie rods, ball joints, and other front end parts there is no substitute for a BFH. Often a 20 oz hammer will do with two blows what a 5 ton puller won't touch. The hammer blows are not directed to the male, but to the outside of the female taper. The hammer blows flex the joint enough the taper pops free.

That may work for your tapers if you can get a good hit on the coupling with your puller installed. A rapid but careful heat of 250F would help and shouldn't harm the plastic.

 

[hickstick_10]

A tapered shaft/hub is a common feature of shipboard machinery in the Navy from li'l guy on small pumps to 22" ************ couplings in propellor shafts. I done lots of 'em.

Usually these tapered fits are set by the designer establishing a "draw" specification intended to result in a certain hoop stress in the hub. Let's asume the permissable hoop stress is 35,000 PSI (Yield strength for unheat-treated medium carbon steel).

You say the taper is 0.1" in 4 1/4" that's about 0.282" per ft. I assume your estimation of taper was just that. 1/4" / ft is a common shallow taper.

OK 4" dia x yield / young's modulus = 0.004." 35,000 PSI x projected area of fit (4 1/4" x 4" x Pi) = 930 tons (about) x 0.3 for coefficient of friction of steel Vs cast iron equals about 280 tons to break the friction. 80 tons is a fart in a whirlwind.

Assuming the taper is 1/4" per ft and it's properly fitted the draw would be (0.004" interferance / taper per inch = 0.192") a hair over 3/16".

The way to remove these taper fitted couplings is to put them under the available tension with a press or a puller then pour the heat to it with a BIG rosebud tip. A soft gas flame is not enough. It has to be an oxy-fuel flame if stationary is capable of heating a spot of solid metal the size of a quarter to red heat in one minute. The heat input to the hub has to be rapid. You have to expand the hub before the heat diffuses into the shaft. Keep the heat moving so the whole hub exterior heats and expands taking the cooler bore with it. Hesitate for a moment and you will hot spot the hub and possibly distort it. Sooner or later there will be a hell of a bang, the hub will jump, followed by an avalanche of puller hardware - mind your toes.

When you re-install the hub, machine the shaft taper so it blues in 80% and marks for the full length. If there is a key, fit it snug but not so tight the hub won't slip past it. Metal to metal almost. Deburr and clean. Heat the hub to 350 degrees F. Arrange a stop to prevent the hub from over-traveling and slip it on. Hold it in position against the stop and wait for it to cool. When cooled on a shallow taper the hub is as good as welded on but it does need some form of axial keeper just in case.

My figures are quick and dirty maybe they should be checked.

+1

You gotta use heat and keep the part under tension, when I get these up to heat, i go all over the female taper with a large aluminum drift ad a 4 pound hammer, tapping all over.

It'l come out, and as an old millwright I worked with said
"Son with sweet oil and patience you can f*** a cat"