Don’t Run Steel in Steel

[Chris CrewParticipant @chriscrew66644]

I had almost completed Giles Parkes’ hob relieving device and have followed his ‘words and music’ to the letter despite having some earlier misgivings about certain aspects of the design but as I had witnessed the original tool in operation I decided the designer knew better than I did. The design calls for a camshaft to run in two bearing brackets with all components made out of mild steel. The brackets had been drilled and reamed together and the camshaft accurately turned. It rotated perfectly smoothly by hand with no binding and was lubricated with light oil. I tentatively set the device up in the Myford and started the machine in the lowest back gear speed of 27 rpm and everything seemed to be working as intended, happy days! However, I decided to let the device rotate for a few minutes to ‘run it in’ and for about 5 minutes there was no problem. Then, there seemed to be some movement of the stationary components developing and whilst I was trying to determine why this should be we had a ‘crash’. The arm that held the change wheel drive rotated and jammed under the main drive gear breaking a tooth on the idler gear and the pin on the telescopic shaft sheared. But why?

When I released the jam and took the device over to the bench to examine the damage and determine what had happened I found that the camshaft had seized in one of the brackets and it was well seized too. After dismantling it took several hefty clouts with a hammer against a brass rod to shift it out of the bracket. Why this had happened has baffled me because the gearing meant the shaft was rotating at only 81 rpm, there was no heat in the camshaft or its bearing and no sign of dirt or grit in the assembly. I can only assume that it was because steel was running in steel, bad practice I know, but that was what it called for on the drawing.

I have since re-drilled and reamed the bearing brackets, fitted brass bushes and made a new camshaft. I will also drill oil holes into the assembly to ensure lubrication. I am in the process of machining out the broken tooth of the idler wheel and will braze in a ‘patch’ and re-cut the tooth. I am still smarting that this mishap occurred but grateful that it hasn’t appeared to have done any damage to the lathe back gear when it jammed and that the damage was confined to everything that can be repaired. And, thinking about it, I may also make a 30 tooth final driven gear out of a ‘plastic’ material as a further safety precaution to protect, hopefully, the lathe back gears against any future possible future jamming.

 

[DiogenesParticipant @diogenes]

Was there en3b involved? ..it is the worst for galling and picking up in my experience..

I’ve cut leaded square into flats before now just to avoid having to use it even for hand-operated shafts to run in.

Gauge plate runs okay at low speeds with MS shafting – and a bit of lubrication.

[Benedict WhiteParticipant @benedictwhite51126]

Chris. you should use a bronze, like leaded bronze for bearings. Brass is not really a bearing material.

[John PParticipant @johnp77052]

Some of the events you have described  were noted  in the original  article,MEW  57   page 44   in  the  paragraph   “Slow drive train”  .The original   was only running  at a mandrel speed   of 10.3 rpm  using an additional  drive motor.

John

[Chris CrewParticipant @chriscrew66644]

“Was there en3b involved?”

It is very possible that the steel I made the bearing brackets out of was EN3B. It had been part of a batch that I scrounged from my erstwhile employer when the tool-room was closed in 2005.

“Chris. you should use a bronze, like leaded bronze for bearings. Brass is not really a bearing material.”

Absolutely agree, but I had a bit of brass of suitable size and it is not as if the device is being used in ‘production’. If I ever get it working I will probably only use it to back of a couple of hobs for the Jacobs gear hobber if that ever get built as the casting have been sat on the shelf for at least 10 years now!

Some of the events you have described  were noted  in the original  article,MEW  57   page 44   in  the  paragraph   “Slow drive train” 

I re-read the paragraph you mention. Mine was not exactly the same sort of issue but I will be taking a few precautions now this crash has happened. The Radford worm-wheel hobber that I have also made uses more of the Myford change gears and any sort of similar seizure may cause even more damage although protection in this case would be provided by the fibre gears in the leadscrew drive train. Even so, I think it may be prudent to introduce a ‘plastic’ gear somewhere in the train and as I will be setting up for cutting a gear for the Parkes device it won’t be too much more trouble to also make one for the Radford attachment.

[Benedict White]

On Benedict White Said:

Chris. you should use a bronze, like leaded bronze for bearings. Brass is not really a bearing material.

I ran a home made lathe countershaft with brass bearings for 10 years without any apparent wear.  Brass will be fine at low speed/light loadings.

Martin

[SillyOldDufferModerator @sillyoldduffer]

Sounds like a design error.  Bearings are almost never the same metal both sides.  Tends to rub, get hot, gall and seize.

Best practice is for a hard shaft to run in a soft bearing shell, two different materials, with lubrication.  Lots of combinations don’t work, such as steel and aluminium.  Others work extremely well, like the hardened steel pivot and ruby found in mechanical watches.  Easier to copy what others have proved works than to experiment.

Steel in cast-iron is OK for lightly loaded slow turning bearings because most cast-irons are full of slippery graphite.  Popular in old sewing machines, many still in good order a century later.

Bearing shells were originally called ‘Brasses’, because they were made of soft brass, the shaft probably being wrought iron, and lubricated with tallow or animal fat.  Brass is a reasonably good bearing metal for steel, and with a little oil will take moderate speeds and loads.  Bronze will take a bigger beating than Brass.

So far, these are all Plain Bearings, which aren’t ideal, even when souped-up with White Metal and a high-pressure oil pump. They work by the shaft riding on an oil layer, avoiding metal on metal as much as possible. Unfortunately metal on metal occur when the shafts in plain bearings stop and start, and  – much worse – if high speed or an excessive load breaks the oil layer.  `

Plain bearings run within their limits take decades to wear out, but running them too fast takes years off their lives.

The main disadvantage of plain bearings is they are high-maintenance.  Gone are the days when a sailor would spend a watch next to a propeller shaft to make sure a bearing was OK, constantly checking the oil, and periodically hosing it down with sea-water to keep it cool.

Roller and taper bearings have mostly replaced plain bearings because they can do high loads and high speed, don’t need TLC,  and are replaceable.

For lightish duty, almost all my home-made bearings are Steel running in Brass with a simple oiling hole that I must remember to keep topped up.  A more sophisticated variant would be an oil bottle with a wick or needle.  A cast-iron bush is an alternative, but I avoid cast-iron because of the dirt.  Oilite bushes are an option too.

Dave

 

 

[John MCParticipant @johnmc39344]

Suggesting that a steel aluminium combination doesn’t work as a bearing is not really right, plenty of steel crank shafts in the automotive world supported on aluminium (alloy) bearings.

Steel on cast iron and cast iron on cast iron can take high loads, slideways on machine tools for instance.

As has been said its all about choosing the right combination of materials.  I steer clear of brass for bearings,  best kept for decoration.  leaded bronzes are readily available in small amounts at reasonable prices and will cover most plain bearing applications.

Stay away from phosphor bronze, good in certain applications but care is needed in choosing for the application.  One application PB was traditionally used for has been substantially superseded by aluminium…

[PeteParticipant @pete41194]

The designer most definitely should have known better Chris.

Steel shafting running on the more common alloys of aluminum as sleeve bearings doesn’t work for well known engineering reasons. Automotive crank shaft or rod bearings can because in that application they would be using a pumped and pressurized lubrication system. So under rotation, the bearing journals are fully supported by the wedge of oil and don’t touch the bearing itself at all other than when the engine is turned off or during the very first few rotations when it’s started. Yes those aluminum alloys are used and they would be a fairly specialized alloy mixed with a few other metals to increase there durability during those engine starts and not one of the more common alloys of machinable aluminum we might have. In the past, the bearing materials were always much softer than any aluminum without any real issues, but even the relatively slow rpm steam engines still worked and supported the bearing journals on that oil wedge. Go back far enough and the use of dead soft Babbit was very common. With mostly random drip fed oiling, any aluminum alloy most of us would have would still be a very poor choice.

Using bronze as low rpm sleeve type bearings often gets mentioned. Unfortunately it’s not quite as simple as that with any non pressure lubricated systems. Most designated as an actual bearing bronze are still a fairly hard material. For good engineering reasons and longest life, then ideally the shaft itself should be hardened. For model engineering purposes that might be of less or of no real concern, but still worth knowing about in my opinion. In most full scale designs, bearings or bushings would be considered the cheapest or consumable item. So it’s still good engineering to have them wear first and then be replaced instead of the usually more expensive or harder to replace component that’s rotating within or on them.

All materials will have what’s known as a coefficient of friction. https://www.engineeringtoolbox.com/friction-coefficients-d_778.html And in general, alike materials shouldn’t be used together due to that serious galling or much higher wear rate issues. Cast iron on cast iron is one of the few where it’s possible and machine tools have certainly used it for a very long time, but it would still a bit less than ideal for the best durability. So hardened ways and even specialized low friction plastics are now being used on the more expensive machine tools. At low rpm and loads, and for most of us, items not being used all that often, then common brass should still work well enough.

[noel shelleyParticipant @noelshelley55608]

Many may laugh but wood can be a good bearing material, it also has other engineering uses ! The straw walker shafts on combine harvesters a few years ago were oil soaked Beech wood and it would be the steel shaft that wore. The riddles were shook by an ash shaker arm.

As for white metal bearings tin is one of the alloying elements and the ML7 uses this type of bearing for the headstock !

As for machine tools useing cast iron on cast iron, consider the relative speed of movement !  Noel.

[bernard towersParticipant @bernardtowers37738]

I am halfway through a sparey diesel which has a conrod in gauge plate with an en8 journal and looking at various write ups people are not recommending it as they are saying that they have have had siezures so have gone the aluminium rod route. perhaps this proves the point. Out of interest I have made mine from 7075 t6 and a nice piece of material it is.

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