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Single Reduction versus Double Reduction Gearbox Reliability 2

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JJPellin

JJPellin

Mechanical
Oct 29, 2002
2,195
I attempted to do a search for my question and was not able to find any relevant threads. I apologize if I missed any.

We are in the process of designing a replacement cooling tower. It is expected to have 6 fans, 18 feet in diameter, driven by 150 HP motors. The cooling tower manufacturer is proposing that we use single reduction gearboxes. All of our existing cooling towers (6 towers with 50 fans) use double reduction gearboxes. The project manager is asking for justification to spend more to get double reduction gearboxes. For clarity, we are talking about right angle gearboxes that have input speeds of 1800 rpm and output speeds in the range of 150 to 190 rpm.

Should we expect to pay a penalty in reliability or service life by using single reduction gearboxes rather than double reduction?

Johnny Pellin
 
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For spur gears, 10:1 is where tooth form mods appear in the interest of pinion life. For bevel gears, probably also. For worms, I don't know.

You should talk to the gearbox mfgr.


Mike Halloran
Pembroke Pines, FL, USA
 
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Thanks, MikeHalloran. The gearbox manufacturer is also the cooling tower manufacturer. They have recommended the single reduction gearbox and this places them in a better competitive position for overall cost on the cooling tower. They claim that expected reliability on the single reduction option is excellent. According to them, single reduction costs less and runs just as well. Since our existing 50 fans all use double reduction, this has apparently not always been the recommendation. The single reduction box will have a single set of bevel gears. The double reduction gearbox would have a set of bevel gears on the input and a set of spur gears on the output.

Johnny Pellin
 
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If single reduction is not new technology to the supplier, then they should have _actual_ reliability data, not estimates, or they should be willing to demonstrate their confidence by offering a service contract for their calculated service life, for nearly nothing.

Just as a frame of reference, do you bother to keep local spares of any sort for the current 50 gearboxes?
Are there local external stocks for immediate delivery, or does someone have to restart an old factory to make parts for you?
I.e., what are your expectations re failure and serviceability?
I don't personally care, so you don't need to reveal anything here, but that's the sort of thing I'd be thinking about, were I in your position.




Mike Halloran
Pembroke Pines, FL, USA
 
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I would venture to guess that the odds that the company you buy your cooling towers from also actually manufactures their own gearboxes are very slim.

They may have a 'cooling towers inc' part number shown on the BOM, but I'd bet they're made by one of the few specialists out there.

If you're near a tower, go look at a physical gearbox and call the company on the name plate and ask them directly.
 
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There is a rule of thumb about the loss of efficiency per stage of reduction. I think it maybe 3%. You could calc the elec cost from this after getting better data
 
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What has the failure analysis of previous gearbox failures revealed.

For a time I worked for a company that offered "predictive maintenance" services and had a bunch of commercial customers. We measured vibration on equipment bearings, did oil analysis, and a bunch of other stuff

My recollection was cooling tower gearbox failures most often resulted from gross lubricant water contamination, maybe exacerbated by infrequent oil changes.
 
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it will be hard to prove which solution would be more reliable since there are lots of parameters involved. design, materials used, roughness of the gears, oil temperature, recommended oil type, ingress of dirt or water and many more all will have some influence on reliability. some of the parameters are controlled by the manufacturer, some depend on local operating conditions and quality of maintenance.
 
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In 30+ years of using many styles of single, and multiple stage boxes, straight, helical, bevel, spiral bevel I haven't had any particular issues with any style.
It's all in things like service factor and knowing what it needs to do.
The spiral bevel rear differential in my pick up has 429,000 miles on it with only oil changes for maintenance.
I've had planetary units radially explode in months because a sales men told my then boss they where just as good as the one we've been using since 1959.
We still manufacture the 1959 version basically and I overhaul about 40 units a year and it rare that we replace gears due to wear alone.
Most damage is from neglect and abuse.
If you are using the gearbox to support the fan blades I'd be looking more at the output bearings than the number of gear sets.


 
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The gears in your truck's rear diff are likely straight bevel gears. And the ring/pinion are hypoid gears.

The MTBF of any gear drive system will be determined by numerous factors, including component manufacturing/material quality, operating conditions, service/maintenance procedures, etc. All other things being equal, a gear drive having more components will have a statistically lower system reliability rate.
 
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Found this on Google

A hypoid gear is a style of spiral bevel gear whose main variance is that the mating gears' axes do not intersect. The hypoid gear is offset from the gear center, allowing unique configurations and a large diameter shaft. The teeth on a hypoid gear are helical, and the pitch surface is best described as a hyperboloid.

Showing my age here but I've worked on cars, Hot Rods, and even a Pro-Street Dragster since the mid 70's when you could work on your car in the garage.
 
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Automotive rear ends have used hypoid gears for a long time. They can transmit more torque, or the same torque in a significantly smaller package, than spiral bevel gears can.

The tradeoff is that they are less efficient. So newer cars are actually migrating toward spiral bevel final drives. Trucks are holding on to hypoid axles because they need the higher torque transmission.
 
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Other ways hypoid gears differ from true bevel gears is that they don't have pitch diameters in proportion to their tooth numbers, the pinion and gear have unequal pressure angles, and they have different profile curvature on opposite sides of their teeth.

Technically, I don't believe hypoids are considered true bevel gears, like straight/zerol/spiral bevels. Don't know about face gears though.
 
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