In-line spur gear train - How to increase efficiency 3

[agdyp]

Dear friends,

I am designing a gear train using 3 in-line spur gears. These gears are supported by 2 parallel plates. I do not have any ball bearings or bushings. Input & output gears have hubs which rotate freely inside the plate holes; whereas the center gear rotates over a fixed shaft.

I have made some prototypes and the efficiency (o/p torque/i/p torque) measured is about 80%. So, efficiency per mesh is about 90%. I really need to increase the efficiency with little to no compromise on load capacity. Speed of input & output gear is about 1 RPM only. Gear ratio is 1.33.

There seems to be a relatively large radial clearance between input/output gear hub od & the plate holes. This would cause the efficiency to drop. I don't know how much though.

I will really appreciate your comments on the following.

1. What are the parameters affecting gear efficieny in order of priority?
2. Is there a standard chart or reference book that gives me exactly how much is acceptable center to center distance tolerance to get over 95% efficiency for a specific gear design
3) What is the minimum surface finish of gears to achieve over 95% efficiency
4) What is the acceptable radial clearance between shaft & bearing surface to achieve over 95% efficiency?

Please suggest if there are other important parameters to be considered.

Thank you.

-agdyp

 

[MikeHalloran]

You seem to have a bad case of "the cutes".

Gear hubs are not ordinarily controlled or finished in a way that would allow their use as part of a bearing pair.

In other words "rotate freely in" is synonymous with "forms a crude bearing with high friction and very limited life".

Is there some economic reason why your design has no bearings or bushings? Especially given that high efficiency appears to be a design goal?

I'm also having a hard time visualizing your basic geometry, i.e., how the gears mesh with each other.
Could you post a picture or drawing, please?


Mike Halloran
Pembroke Pines, FL, USA

 

[Matt51]

For such low speed gears, at 1 rpm, this is an unconventional design.

In order of priority:
1) Friction at the hubs and the middle gear over its shaft. What materials are used? They might be ok, but maybe either bushings or rolling element bearings could be used?
2) gear tooth lubrication - are the teeth lubricated at all, by grease, or are they operating unlubricated? If they cannot be grease lubricated, can a metal with inherent lubricity be selected?
3) gear finish - normally gears used at very low speeds have polishing wear, and the surface finish improves as they wear in. You can superfinish the gears with a process such as REM. It might or might not help.
4) center distance variation - if the teeth move too far into mesh, they have interference. If they move too far out of mesh, they lose contact ratio. Center distance variation needs to be controlled. What are the size of your gears, what is the max possible plus and minus center distance varation?
5) tooth design - finer pitch (more, smaller teeth)improves efficiency. 25 teeth is much better than 15 teeth. Higher pressure angle improves efficiency, 25 degrees is better than 20 degrees. Constructing a specific sliding chart is very informative. As you have such low gear ratio, and one is an idler, profile shift of the cutters will not help you.
6) cycloidal tooth form - is more efficient than the involute tooth form, when there is poor or no lubrication, which can be seen by constructing a specific sliding chart.

 

[agdyp]

Thank you for the responses. I'm using 20dp gears, #teeth=24 on input & output gears and 18 on the idler. Gear material is 1117 steel, hardened to 50Rc. I want to avoid the use of bearings or bushings for economic reasons. Lubrication is not used.

Due to relatively large clearance (about .004 diametrical)between gear hubs & plate holes, the center distances between the 3 gear axis do not stay fixed. The large clearance also enables the axis of the 3 gears to be at an angle and not parallel to each other.

Main goal is to maximize tooth strength and . My recent experiment shows an average efficiency of 88% per mesh.

-agdyp

 

[Matt51]

Hi,

Based on the 18 tooth idler, I would try 25 degree pressure angle gears. This will lower your maximum specific sliding. Or, another way of saying this, you are moving contact away from the base circle.

You can buy sealed tapered roller bearings for car wheels for very low cost at auto parts stores. It might cost less than you think to add bearings.

Or, you can make the side plates out of a bronze, and polish the steel gear hubs.


Positional tolerance of the locating holes must be added to the .004 clearance, to find max cd variation. Depending on runouts of the plate holes and mating gear hubs, you may not be able to reduce the .004 clearance. Crowning the gear teeth can relieve edge loading caused by misalignment.

 

[dimjim]

The higher pressure angle will probably give you more
axial load and increase the wear on your hub and plate
holes. I think the lower pressure angle would be
more efficient but not as strong.

You might consider 25 percent long addendum on your drive gear, no addendum modification on the idler, and consider 25 per reduced addendum on your output gear as a far out 20 degree pressure design system. Special center distances are required.

 

[plasgears]

We used to get away with hard steel pins as gear pins [no bearings]. They were derived from needle bearings. This applied to subfractional power, but we had some operating at higher HP. Our grease was tacky lube with EP and other additives. We added 0.005" to the theoretical pin to pin dimension on pin setting tooling.

 

[unclesyd]

Here is the tension bushing a relatively inexpensive repalcement for roller type bearings in an applications such as yours. I would go for the lubricated type and use as stated above a water resistant high EP grease like LE 3752. I would use the grease on all sliding surfaces even if you don't go with bearings or bushings

http://www.connexusa.com/HTML/application.html

http://www.le-inc.com/products.jsp?productID=237

 

[vc66]

If you need precision, you have to be willing to pay for it.

Roller bearings are not very expensive, and will help you greatly. Also as Matt51 alluded to, positioning of the gears WRT one another is key for efficiency.

V

 

[sacem1]

Forget about trying to increase efficiency if you are not having your gear train over correctly positioned and working spindles (bearings/lubrication/fixed distances), its like trying to get more milage from your car and running it with 20 cement bags in the trunk and low tyre pressure and complaining why you do not get near the EPA estimates.

All other gear changes (pressure angle, high number of teeth and so on correctly mentioned before are useless until you fix the shaft problem.

Regards

SACEM1