Relative merits of different types of mechanical soft-starts 2

[KM]

I want to provide a more gentle start to a machine via some type of fluid coupling or other mechanical soft start mechanism in order to protect the gear train "downstream" of a motor from torque shocks.

The machine is a stoplog lifting crane on a dam. The mechanism, although sheltered from wind and precipitation, is otherwise subject to ambient temperatures -30 to +30 ^oC. It is operated about twice a week year round for a couple of hours to a whole day at a time. The dam is in the middle of nowhere which makes it difficult and expensive to get any maintenance or repairs done.

The prime mover is an antique 1961 GE wound rotor induction motor, 15 hp, 875 RPM with starter and drum controller of the same vintage. The motor frame size is 324Z.

There is a V-belt from the motor to the gear trains, which consists of early 20th century cast gears. A clutch system allows the motor to drive either the hoisting gear train or the travelling gear train (the machine moves along tracks on the dam deck). The motor must deliver power in both directions of rotation. The motor and gears see some plugging and jogging service as the operators have to get the machine lined up just right over the sluices in order to place stoplogs correctly in the gains.

The condition of the gears varies from reasonably OK to really poor. The journal bearings throughout the machine are very loose, and some of the gears farthest from the motor actually have trouble meshing because of all the backlash and general slackness in the system. (We're planning for some gear and bearing repairs, but we won't be replacing all the gears certainly).

I want to protect the gear train from torque shocks after the repairs.

Because of the age of the motor and associated controls, and because of the wide range of ambient temperatures, I'm afraid of the can of worms involved in fiddling around with electric soft starts or VFDs.

As for mechanical soft-starts, I think I have a few options:

Fluid couplings sound pretty good and I've found several suppliers of them, e.g.
-Siemens' FLUDEX
-Voith (but they seem to supply only really big stuff not the little things I'd need)
-Kraft Power's Transfluid
-Baldor-Dodge's Paraflex
These sound good, but I'm worried a bit about the wide temperature range and how the viscosity of the oils used in them would react.

Centrifugal clutches sound like an option that may be less sensitive to temperature, but they seem to be more popular for automotive applications than industrial ones. The suppliers I’ve found are:
- Hilliard's Twiflex or Linkless centrifugal
- Noram
- BLM

Baldor-Dodge's Flexidyne is like a fluid coupling, but uses pellets of steel shot instead of oil. This seems to be a real Goldilocks solution, but I really know nothing about them beyond what I read in the product literature. Does anyone know of other manufacturers who offer this type of device?

I'd like to hear from folks who have experience with mechanical soft-start devices what they have found to be the relative merits of centrifugal clutches vs. fluid couplings (vs. other alternatives?)

Also, I suspect that having such a soft-start device installed may make it slightly more difficult for the operators to get the machine lined up properly over the sluices, as by its very nature it would impair their ability to plug and jog the motor. (We may have to supply them with some type of pry-bar so they can shift the machine along the tracks for fine-tuning the position.) Any other implications that I may not be aware of?

 

[Jboggs]

My gut tells me the Flexi-dyne is a very attractive option for two reasons: (1) it seems the most impervious to the environmental challenges (heat, water, dust, etc.), and (2) its been around for a long time, which tells me it is reliably filling a need somewhere. I would strongly recommend you get in touch experienced application engineers (NOT SALES) at all the manufacturers you are considering and pick their brains. That's what they are there for.

 

[electricpete]

Don't the Vee belts already provide a certain degree of the type of protection you're seeking?

Do the belts squeal during start now? If not, perhaps loosen just enough to squeal during start. The worst you can do is wreck belts and sheave groove... these are easily replaced components. While I'm not crazy about that idea, it might be better than adding complexity to an already complex one-of-a-kind system.

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(2B)+(2B)' ?

 

[KM]

Electricpete - the V-belts probably do provide some softening, but not enough as evidenced by the large numbers of gear teeth with chips out of them! Loosening the belts is a good idea for the interim, certainly---and thanks for the idea.

 

[electricpete]

No problem.

The worst you can do is wreck belts and sheave groove

Let me clarify that. Of course the most likely result of decreasing tension below manufacturer's recommendation is just increased wear rate and shortened service lift on belts/sheaves. For my own peace of mind - I'd like to add the worst scenario is probably a belt breaking and hitting someone standing nearby if belt guard is not present or inadequate. I'm sure you can think of ways to manage that risk such as keeping people away especially during start.

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(2B)+(2B)' ?

 

[DRWeig]

Nicely stated, electricpete.

Best to you,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[Tmoose]

Hi KM,

Are the gears supported by loose bearings showing the tooth damage?
Do you have pictures of the type(s) of tooth damage?

I don't think I'd necessarily blame the starting conditions for tooth damage on 80 year old gears with supports so worn that have trouble meshing.

 

[Artisi]

Just a nonsense posting, why would you even consider soft-start of any sort to overcome the problem of old antiquated worn-out and damaged equipment expected to perform as new in the condition as described, [highlight #EF2929]"The condition of the gears varies from reasonably OK to really poor. The journal bearings throughout the machine are very loose, and some of the gears farthest from the motor actually have trouble meshing because of all the backlash and general slackness in the system"[/highlight]

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[ScottyUK]

"I want to protect the gear train from torque shocks after the repairs." (my underline)

What about one of the magnetic couplings from (e.g.)

http://www.magnadrive.com/?

I don't particularly buy the energy saving claims for their variable speed drives, but the couplings are robust and simple. Catalogue at this site.

 

[MintJulep]

Motorcycle clutch.

 

[zekeman]

Unless you get the backlash down considerably, the soft start will only help partially. Shock comes from velocity and backlash. Softstarting only helps reduce velocity at impact.
Given the very low usage factor,if you do a decent job of refurbishing to remove most of the backlash, I would think you can get another 50 years out of the gear train without resorting to any special starting methods.

 

[KM]

We certainly plan on repairing the damaged gears and the bearings. But the whole machine is old, and even AFTER the repair I want to be more gentle on the gears.

The electrical arrangements have been changed over the years in order to allow operation by 2 instead of 3 operators---the original drum controller for the wound rotor motor had a man controlling the speed from 0% to 100% with that control. They've now set that speed controller at half-speed and operate with an on-off pendant controller from the dam deck. So every time they start, they get a torque-shock through the gear train. There is no way they can go back to 3 operators, so I'm stuck dealing with this on-off control. Given that the application is outdoors and the winter temperatures are low, and given that the existing motor and starter date to 1961, I'm afraid that a VFD would be an awkward can of worms. Hence, I was looking at some form of mechanical soft start to be incorporated, AFTER a repair of the gears and bearings.

Thanks for the lead on magnetic drives, ScottyUK, I'll look into that.

 

[1gibson]

Sometimes a can of worms is better than no can at all. If you could get a modern VFD and program it with a "ramp up" speed profile over a couple seconds, it would reduce the shock loading, make minor adjustments easier, and have negligible impact on long distance travel.

 

[dgallup]

Sounds like you need to make that "original drum controller for the wound rotor motor had a man controlling the speed from 0% to 100%" remotely operable or have it automatically go from 0-50% over a period of time.

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[KM]

1gibson - I just don't like the idea of a VFD outdoors with -30 to +30 C conditions and no control on relative humidity. Others have warned me away from them for this application, and I take their experience into consideration. Have you had problems with fluid couplings or similar that make you favour VFDs for this application?

dgallup - I agree that I need to ramp up the speed from 0-50% over a period of time, in fact, that's what fluid couplings are supposed to do.

Are these things so rare that no-one has any actual experience with them?