Gearboxes - Can this speed reducer work as a speed increaser? 3

[nanobot29]

I'm looking to purchase a used 4,000 hp gearbox with a single stage helical gears. It was designed to be a speed reducer but I'd like to run it as a speed increaser.

Gear ratio - 14:1
low speed shaft - 1,800 rpm
high speed shaft - 25,200 rpm

the gearbox is single direction. Can i run this as an increaser if i maintain the direction of torque and rotation? i want to be able to drive the 1,800 rpm shaft (at 1800 rpm) and obtain 25,200 rpm as the output.

I did a quick free-body diagram and the thrust loads would remain in the same direction as if used as a reducer. What else do i need to consider? Are mating pairs of helical gears ever machined such that the pinion can't act as the driven gear?

if i inspect this gearbox myself, what should i look for in the gearing?

thanks for any advice,
nick

 

[MikeHalloran]

Take an oil sample and sent it out for analysis.

Call the gearbox manufacturer and ask them to bless your misapplication or propose alternatives.

What do you do when this gearbox frags? Do you have a source for more of them?



Mike Halloran
Pembroke Pines, FL, USA

 

[drawoh]

nanobot29,

25,200rpm sounds fast to me. You could have problems with centrifugal force stresses, and maximum bearing speeds. What does the manufacturer think?

--
JHG

 

[nanobot29]

Mike,
What would i be looking for in the oil sample? The company that manufactured this gearbox was acquired by a company in Germany. The design is 12 years old and the new company is not familiar with the design intent of this gearbox. They prefer not to advise on running the gearbox as an increaser. i asked them for drawings of the gears and bearings so that i could analyze the drive train but I'm not sure if they would be willing to provide that information. Thanks for the input.

JHG,
The gearbox is rated at 25,200 at the high speed shaft and 1,800 at the low speed shaft. we would be operating within the speed and power requirements.

 

[MiketheEngineer]

Buy two - one for backup!!! And when you burn out the first one - buy another

 

[zdas04]

I'm thinking about the reduction gears on a Navy ship. It takes a LOT of hp to push a ship, but that hp is passed through the same kind of gears you are talking about. I've been on the throttles when we went from flank speed to full stop (so the movement of the ship was trying to drive the propeller) and the reduction gear took the drive/driven shift perfectly well. I don't know if this example applies (since forces from either direction were part of the design criteria for the reduction gear, but we don't know that they were part of the design criteria for your unit), but it seems germane.

One thing that was important in those huge units was that the shape of the gears tended to pump oil from the outer edges to the groove in the center of the gears. When going backwards (or when the screw is driving) the tendency is to try to pull oil from the central groove (where there is minimal oil) towards the edge. I think I remember (from 1977) an issue if you went in reverse longer than a set period (it was days, not weeks as I recall). You might look at the gear lubrication system and see if you can see a lubrication issue running backwards.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[redpicker]

It is not clear if you application is to drive the speed reducer backwards, as zdas04 has commented on, or just apply power to the input end in the same rotational direction as the uint was designed. While in theory, it shouldn't matter if you are maintaining the same rotational direction (after all, torque transmission is torque transmission). In reality, however, there are bushings, bearings, and couplings that are designed to accomodate differences in alignment in the input and output shafts. The alignment of these shafts may change by using it as an increaser rather than a reducer, hence the advice to have some back-up uints handy.

rp

 

[Metalguy]

4,000 HP and 25k RPM? Sounds like something from C-130 (Allison T-56). Just guessing---.

"You see, wire telegraph is like a very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? Radio operates the same way: You send signals here, they receive them there. The only difference is there is no cat." A. Einstein

 

[rmw]

Do a tooth contact test (with bluing or the proper paste) on the leading edge of the teeth in a "normal" direction and then do it in the "reverse" direction you intend to operate it. If the contact patterns are the same for that part (irrespecive of other points made above about the lubrication distribution) you are OK on that point. However, if the contact point in the "revrse" direction is not the same as the "normal" direction you have problem.

rmw

 

[geesamand]

If the original design was 25k in and 1800 out, and you're going to run it at 1800in and 25k out, this may be reasonable.

If the original design was intended to be 1800in and 125 out, I wouldn't try it. The bearing selections probably can't handle operating 14x faster, and bearing settings, lubrication design, and lubricant selection will be all different.

As for inspecting the gearing, basic inspection for water damage, pitting, scoring, abrasive, etc on the teeth.

David

 

[zdas04]

In the original post the OP said that he was keeping the gears rotating in their original direction, "just" swapping the driving and driven ends. We have gone a bit into the weeds (sorry if my Sea Story derailed the discussion, I was really talking about thrusts in driving vs driven and got off on a reverse rotation tangent).

Having dealt with a few gear manufacturers there are two things I would say: (1) it is unlikely that any manufacturer would warrant these gears in this service; and (2) it will probably work because of the safety factors normally built in.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[nanobot29]

Thanks everyone for the input.

My intent was, as zdas04 mentioned, to operating the gearbox in its intended direction and speed... except i will be driving the low speed output shaft. Internally, the thrust loads will be in the same direction. The only difference will be that the tooth contact surfaces will change.

I will look into trying a tooth contact test as rmw suggested and possibly checking oil samples for metal shavings.

 

[fixermaker]

There is one more consideration. The reverse direction may not handle the torque increase unless you are significantly below power limit. If you start with 14:1 ratio you will increase the torque by the same fold. We changed a gearbox and motor on a rotating assembly from 3/4 to 1 hp the increase in start up torque that occurred from an increased power ( same ratio's) broke the motor mounts.The 3/4 hp motor actually only worked because it was not able to produce that surge in torque causing it to spin up slowly. So for you you may need to consider more than does it work backwards. You may have to create a VFD type drive to ramp up the speed rated for your motor, 1800 RPM sounds like electric . Most likely your gears will fail near the input shaft and jump. depending on your application you might just want to opt out for a chain style increase system.

 

[kirrer]

I may be missing something here, but your original statement that the thrust loads are in the same direction doesn't make sense to me. If I have just two helical gears in mesh running in the same direction, torque from gear A will produce thrust loads in the opposite direction as thrust loads caused by torque from gear B. I drew the attached diagram to show what I meant. Notably, if you have a force driven on the opposite flank as is designed, the thrust direction for each separate gear will reverse.

 

[RolMec]

Highend gears (the more if from some reputable supplier and with a high power/weight index) will have subtle yet most important gearing improvements, notably different crowning modifications. If now this gear was designed for one single running direction, most probably the teeth shall not have these improvements on their other, not primary load bearing side. The reverse will imo not work in the long run. Then again the load bearing capbility of such gears is highly dependent on lube conditions, see replies above. You might be looking at a refurbishment of gearbox inner installations.
Warranty???
R.

 

[rb1957]

"The reverse direction may not handle the torque increase" ... am i missing something?

the original gearbox was a speed reducer, output speed = input speed/14, (and a torque increaser, output torque = input torque*14, yes?)

the proposal is to reverse the input and output shafts so that the output speed = input speed*14 and the output torque = input torque/14, no?

i don't see a "torque increase" ?