Shaft steels

[agpowder]

Hi, I hope someone can help me.
I have designed some specific application machinery and one of the processes is a twin screw extruder for making powder coatings, I designed this machine two years ago, made it and have been using the machine now for over a year. originally it was only meant to be a prototype just to see if it worked, ie did it make powder coatings, anyway it did and I have developed a business around this extruder.
The problem is now the gearbox, the shafts last about 2 months and then fail. I recently redesigned the mechanics basically beefing up the shafts diameters etc. My question is: Previously (whilst living in England) I always used En24T, turned the shaft, installed and ran and I always had good luck with it. Here in the States I have had trouble with steels, Does anyone know a decent equivelent to En24T (is it 4340 - chemically similar but is it equiv in physical properties?) I also used 1144 on advise from our steel stockist, but I think he had another motive. OR - does anyone know where I can get En24T from in USA?

 

[metengr]

I would consider 4340 low alloy steel, quenched and tempered to strength properties similar to the original shaft material (En24T). The 4340 low alloy steel has excellent hardenability, good notch toughness and good fatigue strength.

 

[btrueblood]

1144 is resulphurized steel, great for machinability, but generally poorer than the equivalent low-carbon steel (1044) in fatigue life. The sulfur inclusions that make the steel have small chips in machining also act as stress risers that reduce its fatigue properties.

 

[ctopher]

I have always used 17-4 for any type of shaft.

http://www.specialtysteelsupply.com/17-4ph-stainless-steel.php

Chris
SolidWorks/PDMWorks 08 3.1
AutoCAD 06/08
ctopher's home (updated Jul 13, 2008)

 

[agpowder]

Thanks for the answers.I am going to try elaborate the problem. The gearbox has one shaft in (Drive) and two shafts out and because of the center distance of the twinscrew extruder bit I am very limited on space,ie, diameter of shaft and/or bearing diameter. The twinscrew extruder is also a high torque application and fairly slow speed.
In the perfect world that hasn't arrived here yet I would just over engineer the shaft diameters and probably have enough to not have an issue but I am struggling for diameter, this is why I would like to use the best steel available for low speed high torque applications? Like I said earlier En24T is my fall back but I did use this material from advise of chartered engineer etc but that application was a high speed low torque one. Maybe the material should or could be different??
I really appreciate any advice.

 

[metengr]

Does anyone know a decent equivelent to En24T (is it 4340 - chemically similar but is it equiv in physical properties?)

4340.

 

[EngineerTex]

I agree with all the posts, but I would also throw in a couple of things.

1) How is it failing? Is it bending? Is it breaking? Where is it breaking? If you have machined a sharp corner into it or have a rough finished surface, that's going to have a HUGE effect on the life of the shaft.

2) The manufacturing process of the bar stock also has a lot to do with your strength. Is it cold drawn? Is it hot rolled? Is it forged? This step in the manufacturing process will change the strength and toughness of the steel as much as the heat treat will.

3) Your supplier may supply the shaft in any number of conditions. AISI 4340 can have an extremely wide range of strengths, depending on the temper. You should find a supplier who can offer the shaft in a fairly wide variety of hardnesses. For example, here's a link to a list of a few different types of steels, but notice that 4340 comes in a couple of different grades -- 66ksi yield for plain 4340 and 100ksi yield for what they're calling "aircraft alloy". This is achieved mainly by a different tempering, but look down the list and you can find 4330 aircraft alloy that has a minimum yield of 185ksi. The difference between 4330 and 4340 is a difference of 0.10% carbon in the chemistry, but that change doesn't account for the strength difference nearly as much as the heat treatment does. Anyway, the point being made is that two shafts made of 4340 may have wildly different strengths and you need to be armed with this information when you talk to your supplier.

http://www.emjmetals.com/esl/

About the 17-4PH -- you can get some really good results with this, but it's hardened in a different manner than 4340 and you still need to make sure that you're getting what you want for your application in terms of minimum strength and toughness.

So, after going through all of that, I would pass that along with the caveat that you need to remember that the harder you go, the more brittle the material. Just something to keep in mind, but if you don't have sharp corners and stay away from the rough surfaces and aren't subjecting the shaft to sudden impact loading, you might just ask for a harder condition of the same material you've been asking for all along.

-T

Engineering is not the science behind building things. It is the science behind not building things.

 

[israelkk]

No one does any strength calculations any more? The shaft material and heat traetment selection should be selected based on the torsional and bending analysis of the shaft. This is quite a simple and straight forward calculations. No need for FEA. I do not see any point suggesting a candidate material without tackling the stresses involved not to mention stress concentration points and fatigue analysis.

 

[EngineerTex]

I agree up to a certain point, israelkk. But the OP said that he was limited on space and couldn't go any bigger. Since it's failing, I assume that calculations would show that something is coming up short. But we don't know what, which is why I asked how it was failing. If it is bending, then it's probably not hard/strong enough. If it's failing in fatigue, it might be a surface finish or a stress concentratin problem. If it's experiencing shock loading, then the shaft he has may be too hard/strong.

I agree that there's no substitution for hand calcs, but you could easily show a calculation that you could drop lots of weight off of the frame of a car by making it out of tool steel, but I'm sure you would agree that that's not necessarily the best solution.

-T


Engineering is not the science behind building things. It is the science behind not building things.

 

[eromlignod]

If it takes two months to fail, it sounds like fatigue. I would take a look at your design to make sure that you eliminate notch sensitivity, etc. Fillet all your interior angles and have the shaft turned, ground and polished (or buy it stock that way).

Don
Kansas City

 

[GregLocock]

You could build a car out of tool steel and remove a lot of weight, but it would fail many tests/requirements other than durability. A car is a very bad example.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[desertfox]

Hi agpowder

I think like Israelkk and EngineerTex you need to establish the loads on the shaft first and then select a material based on stress calcs and the enviroment.
If your short on space that will also drive material selection.
Can you give details of shaft sizes and loads, then we may be able to assist you further.

regards

desertfox

 

[agpowder]

Thanks for the help.
The shaft failures that were happening every 2 months or so were either bearing race impaction on the shaft,ie diameter reduction from bruising under the bearing race or the shaft would snap at one of the machined shoulders.
Recently I have upsized the shafts (at the expence of the bearing sizing)
The machining work is done by me, I am completely self tought on lathe/ mill and whilst the tolerencing is good the finish and the make quality is probably sub standard for what I am expecting the machine to do now.
The machine failed last afternoon with the bigger shafts but I think I have identified a problem inside the extruder (putting a whip load of the shaft). I am working now to eliviate this problem.
I am begining to think the machine needs some design and manufacturing TLC that I am not capable (or qualified) to do.

 

[agpowder]

Engineertex: your second question, I just buy 1144 from McMasterCarr, machine it and install it.
Desertfox: The shafts are 213mm long, bearing journals are 25dia and 20dia. Spur gear sits on a 21mm dia. The gear is 12 pitch 1.667" Pitch. The shaft has a 5/8" hexagon that I machine onto the end to drive the extruder screw. The hex is 32mm long. The two output shafts are fixed at 1.57" center distance. I have to stagger the driven gears on the two shafts to have everything fit inside the gearbox.

 

[SincoTC]

As mentioned by EngineerTex and others, it is very important to make sure that the "machined shoulders" do not have sharp corners, they should be almost as large as the the radius on the bearing's inner ring (not bigger though or the race will not be located properly against its end face). The surface of this "fillet radius" must be smooth, without scratches or chatter and should blend smoothly to the journal and shoulder without ridges.

Trevor Clarke. (R & D) Scientific Instruments.Somerset. UK

SW2007x64 SP3.0 Pentium P4 3.6Ghz, 4Gb Ram ATI FireGL V7100 Driver: 8.323.0.0
SW2007x32 SP4.0 Pentium P4 3.6Ghz, 2Gb Ram NVIDIA Quadro FX 500 Driver: 6.14.10.7756

 

[agpowder]

Desert Fox: As far as loads are concerned, I have thought about this a lot over the last two years and I think the perfect situation the load would be an axial load only where tha product in the extruder transfer the load axially back into the gearbox. I installed a thrust bearing on both output shafts for this. The process screws and kneader blocks of the extruder intermesh with each other and sit into the extruder barrel. I have two 2.060" diameter holes on the 1.57" ctr. The two holes overlap (screws intermesh) The individual screws have a diameter of 2" and should freely rotate in the barrel. The barrel/screw is continuously lubricated and float in the molten product (powder Coating plastic)The screws are 40" long.
I think the screws do whip and this is what is giving the failing load on the gearbox. I do not know how to "work out" this load other than recognise that it can happen especially if the machine runs empty (it shouldn't do this but sometimes does because operator not filling hopper and when the machine has been repaired (and is clean))

 

[agpowder]

SincoTC: I use Tungsten turning inserts that are radiused at slightly less than the relieving radius on the bearings

 

[eromlignod]

Is there any way to eliminate shoulders altogether? I don't know what your shaft design looks like, but there may be ways to design using clamp collars, snap rings, tapered cotter joints, etc.

If you don't have a thrust load, you might even consider letting the shaft "float" in the inner race of the bearing with no shoulder.

Don
Kansas City

 

[desertfox]

Hi agpowder

I think you might find that you also have torsional loads on the shafts in addition to axial ones.
Failure of shafts at shoulders is a known weak spot due to stress concentrations.
By the way when the shaft fails at a shoulder what to the failed edges look like eg:- can you push to failed surfaces together like a jigsaw or is there to much distortion, also are the faces of the shaft parallel to the shaft axis or slightly helical along the shaft axis.

regards

desertfox

 

[Tmoose]

" bearing race impaction on the shaft,ie diameter reduction from bruising under the bearing race "

What is the fit between the shaft and bearing inner race?
It probably should be ~ 0.0005 inch diametral interference, with a ground finish.

If its a ball or roller bearing, and the arrangement is the fairly typical stationary-outer-race/shaft-and-inner-race turning-together with a fairly constant direction radial load, the tendency for the shaft to creep ("spin")inside the race is strong if there is any clearance between race and shaft. At a mere 100 rpm there will be about 1 million rotations in a week (24/7 service). If the loads are high then even a line-to-line zero clearance fit will wiggle then creep.