Changing center distance in spur gears 1

[dmason83]

Hi, i'm currently looking at a problematic gear train on a machine my company made. It's being used in a roll-forming application and the gear teeth are snapping clean off at the root of the tooth due to the fact it is under-designed (i did not design this).

Currently: 14.5 degree pressure angle and center distance between 2 gears changes a total of .133"

Some specs are being changed so that the center distance change is only .075".

I'm wanting to make the gears 20 degree pressure angle. I am also enlarging the face width as much as possible (dimensional limitations). Does anyone know how this will affect the backlash on the gears and any other important aspects of the operation?? the gears are operating at approximately 10 rpm.

Material change should not be a factor. Astralloy-v is being used and is doing better than 4340HTSR performed.

 

[automatic2]

If your not already, you should be seriously consdering a herringbone. Increase your load carrying surface.

 

[dmason83]

automatic2, I thought of that when they first started failing. only problem is the gear box is so tiny it may be impossble to install the helical gears. There are many bad design areas to this machine and has caused nothing but problems so far.

I will look into the helical gears further to double check that route, but i fear it will not work.

 

[dimjim]

When you say the center distance change is .133
and you want to change it to .075, are you talking
about an increased center distance i.e. long addendum
gearing? The 20 degree gears would be stronger.
You might also just decrease the number of teeth of
each gear by 1 and cut them on the same gear blanks
to increase their strengths and operating pressure angles. Do these have full fillet radii?

 

[dmason83]

dimjim,

no, it is not a long addendum gearing, just change of centre distance during operation/setup.

the center distance depends on the material gage being used.

for a job, one gage thickness will be used and the machine will be set. example center distance of 5.5", but due to un-uniform sheet metal we may have to change the centre distance by +/-15 thou during the completion of that job.

Then another job will come around and the machine will be set at say 5.545" centre distance.


No, the current gears do not have full fillet radii.

 

[drawoh]

dmason83,

I have been confused about this centre distance stuff.

Gears are mean to work at their nominal centre distance. If you cannot maintain that distance accurately, your driving torque goes way up. This affects your power and efficiency, and it ought to affect the force on your gear teeth.

Can you build an idler gear into your system? A moveable idler would allow you correct centre distances. That might just solve your problem.

JHG

 

[dmason83]

drawoh,

the moving idler wouldn't work.

the 2 gears where the centre distance is changing are mounted to shafts with tooling on it. one shaft is rigid, the other shaft must be adjustable to allow different sheet thicknesses to be processed, therefore the gear must move aswell.

DM

 

[drawoh]

dmason83,

I understand that you have a shaft and mechanism that moves. You need some way to ensure correct engagement of the gears. A pair of idlers can move separately from the adjustable shaft and gear, and rotate the gear in the same direction as before. When you raise or lower the shaft, you reposition the idlers to get the correct engagement.

Another alternative is a heavy duty chain drive, again with an idler.

I designed a zero backlash gear drive once where the gears were spring loaded together. Aftwards, the gear fabricator told me I should have asked him to cut one set of gears oversize so that the two forms would mesh properly. The involute gear form has to be accurate to work.

JHG

 

[dimjim]

What specifically are the number of teeth in the gears?
Module or DP? Thanks for the description of increased
center distance to control thickness of the rolled parts.
Can you make two or more passes to keep from breaking your
existing gears? Backlash spread is a function of the sin
of the pressure angles if that is of any help to you in answering the backlash result of changing the pressure angles of the gears. Yes, increased tooth length and increased pressure angles plus full fillet design would help incrementally. Each is a step in the right direction. What is the core harness of these gears?

 

[dmason83]

drawoh, i wasn't thinking correctly when i answred your previous question. but still, there is no room in the gear box for such and application. thx for the input.

dimjim, the driving gear is 4.5PD, 4 DP with 18T, and the driven gear is 6PD, 4DP with 24T.

There are currently 6 passes to get the profile we're after but like i said before, everything is underdesigned. The 4th and 5th passes are doing most of the work and thats where the gears are breaking.

The gears are air hardened through to 321-388 BHN

http://www.astralloy.com

if you're wondering, there is an estimated force of around 25000lbs (tangential) on these gears.

DM

 

[drawoh]

dmason83,

Is that 25000lb from the torque, or did you take into account the impact loads or whatever it is when you do not mesh them properly. All this helps is your calculation.

My gut feeling is that you have an impact load rather than a high force. A more impact resistant material like bronze or a reinforced plastic might work better. Could you assemble one gear out of radial pieces with tangential spring loading?

I hate kludges. Is the thing your boss is trying to preserve really worth it? I would not want my company's reputation to stand on a design concept like this.

JHG

 

[dmason83]

That high force is not due to impact loading. Impact loading would make it greater and if the machine is operated correctly there should be smooth movement at all times. It is actually the force (approximated, not calculated)that may occur just from the drive system. 4Hp variable speed electric motor with 444:1 inline reducer.

No, our company does not rely on this machine. We have been designing, manufacturing and selling our successful equipement for over 100 years now.

The whole story behind this one machine...a company was using this machine and it was getting old so they wanted a new one made. They asked us to duplicate their old one and thats what we did with a few minor adjustments making it more user friendly. We put no design into it because the job didn't call for it.

It has caused nothing but problems around here since limited adjustments can be made to make the machine capable of its duties. It doesn't help when the customer uses 5.2mm when the old machine was only designed for somewhere around 3.4mm.

This has also brought to attention that they need a machine that is capable of this kind of operation. The money man of that project is thinking about getting us to design a new machine. hopefully this happens.

DM

 

[dimjim]

Before I do any rough calculations,
can you use 17 and 23 teeth on the same
gear blanks, or do you need that specific
18 and 24 teeth combination? I also assume
you are using full depth gears. You would
use the same center distance for these new
gears. I assume the 18 tooth gears are
breaking before the 24 tooth gears break.
If so, it may make sense to use a 60 percent
long on the pinion and only 40 percent long
on the driven gear. You could also try using
just a 17 tooth pinion instead of the 18 tooth
pinion gear blank and see how that combination
works for you. Keep us informed on what you
do to resolve this.

 

[strokersix]

I'm picturing a conventional slip roll machine with three rollers, correct? If so, there should be a relationship between roll diameter, slippage between rolls and material being worked, and gear tooth count. If that relationship is off perhaps that is causing excessive gear tooth loading.

Just a thought.

 

[dmason83]

Strokersix, your on the right track for your visualisation. picture a 2X5 array of shafts. 5 stations each with an upper and lower shaft. each has respective tooling on it forming the sheet metal more a each pass.

Yes, there is a relationship between the tooling diameters and the gearing dimensions minimizing any chance of excessive impact/slipping loading.

DM

 

[MikeHalloran]

I thought he was talking about a linear machine with multiple paired rollers having different shapes, as is used to form continuous rain gutters from coil stock.

In which case it would make sense to use a drive like steel rolling mill, i.e. double output gearboxes with the outputs on fixed centers driving adjustable roll stands through short shafts with universal joints.

It sounds like this particular machine was built on the cheap, omitting the drive shafts and assuming that it would always work with one stock thickness, so minor thickness adjustments are made by adjusting the gear lash ... but in this case, they have to separate the gears too far in order to get the thickest stock through, so the gears are meshing mostly on the addendum.

... which should cause excessive wear of the addendum, but not necessarily fracture at the root, which brings up the obvious question:

Has anybody checked the profile of the rolls to make sure there's clearance for the thickest stock at each station?



Mike Halloran
Pembroke Pines, FL, USA

 

[dmason83]

Dimjim, I wouldn't bother with any calculations at all. I'm just looking for the strongest working solution for this problem since even when that is found, it will probably still be underdesigned.

Dropping a tooth on each gear. I don't see a problem with that if i stick with spur gears, but I am most likely going to go with helical gears due to their advantages. After talking to the guys in the shop, it is possible but annoying to get helical gears in the gearbox.

DM

 

[dmason83]

MikeHalloran,

You've pictured perfectly our roll-forming machine we manufacture for the corrugated steel piping industry. If we get the go ahead for a new design of this machine i'm looking into using that setup.

I agree completely with you that this machine was made on the cheap side although when smaller gages are being formed it works perfectly.

Whenever we have had the machine in for a repair or when it first went out we checked each station for clearances between the tooling and it was good. Although these clearances can be changed on the fly during operation. Sometimes it's needed (ununiform sheet metal thickness) and sometimes the operators decide it's needed (this can lead to problems obviously).

DM

 

[dmason83]

Was just talking to my boss about a few of these ideas. To his knowledge both 20 degree pressure angle spur gears and helical gears will not make this any stronger. This is because the gears are operating so slowly (approx 10 rpm).

Does anyone have a rebutle to what i've just been told??

 

[drawoh]

dmason83,

I was wondering how effective 20[°] gears would be operating that far off the base circle. All the diagrams in the text books show gears properly engaged. A precisely fabricated and assembled set of involute gears engage multiple faces continuously, and they run smoothly. If you separate the faces, the action should be more complex. There ought to be a lot more friction, and any non-constant motion should be creating loads over and above anything predicted by conventional gear calculations.

With this weird geometry, 14.5[°] gears might work better.

You could spend a lot of time trying to figure this all out. This time might be better spent just re-doing the thing properly.

JHG