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Speed Reducer Question

03/20/2017 3:34 AM

Hey its my first year as a mechanical engineering student so I don't know if this is the right forum as I'm not a professional... if not redirect me to a student forum lol.

We were instructed to design a speed reducer and do simple calculations for it.

I chose to go for a bevel configuration, I guess something that looks like this but with helical teeth.

The requirements are: module of 6 mm, average tooth width must be 12 mm, shaft diameter of 12 mm and pressure angle of 20 degrees. The axes of the input and output shafts must intersect at 90 degrees (perpendicular). Also the speed ratio must be around 120.

I guess my questions is I don't get how to get a speed ratio of 120...I mean I get that I have to pick number of teeth of input/output that would give that ratio but HOW LOL. I mean from this link speed ratios for bevel gear configurations are very low so how do I achieve this? http://khkgears.net/product-category/bevel-gears/

Would I also make one of this bevel gears a planetary gear?

Also could someone explain what a pressure angle is for two perpendicular gears?

Thank you

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#1

Re: Speed reducer question

03/20/2017 5:24 AM

The 120:1 ratio is impractical in a single stage of bevel gears. (For instance, your sketch is about a 2:1 ratio.)

A combination of planetary gears, or better yet a cycloidal drive, might be more feasible, but even they might need more than one stage.

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#2

Re: Speed reducer question

03/20/2017 5:51 AM

For a single stage as shown, the speed ratio is just the ratio of the number of teeth in each gear, so (as Tornado pointed out) - it's not practical in such a configuration. You could just about do it with a very large ring gear and a small pinion, but that would mean a much larger enclosure.

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#3

Re: Speed Reducer Question

03/20/2017 8:09 AM

Maybe a small gear driving a big gear. The big gear is on a shaft with a worm which drives another gear. This can be designed so that the input shaft with the little gear intersects at right angles the output shaft with the gear driven by the worm.

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#4
In reply to #3

Re: Speed Reducer Question

03/20/2017 9:25 AM

Let the kid take note that the driver and driven is not reversible.

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#5
In reply to #4

Re: Speed Reducer Question

03/20/2017 9:35 AM

But as it's a speed reducer - that's not really relevant.

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#11
In reply to #3

Re: Speed Reducer Question

03/21/2017 5:44 AM

Seems those are scew not perpendicular. The centerlines of the shafts won't intersect with a worm gear, will they?

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#12
In reply to #11

Re: Speed Reducer Question

03/21/2017 6:16 AM

Axes are perpendicular (at 90º to each other) but not in the same plane (i.e. not intersecting (as required)).

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#13
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Re: Speed Reducer Question

03/21/2017 7:47 AM

Perpendicular typically describes the relationship between two intersecting lines. Skew lines do not intersect and as such shouldn't be considered perpendicular.

While you can imagine vantage points for which a 2d perspective would look perpendicular, there are far more vantage points for which that is not true.

.

The quality you seem to be describing is 'orthagonal'; parallel to a line that is perpendicular to the line referenced.

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#14
In reply to #13

Re: Speed Reducer Question

03/21/2017 8:10 AM

This is what the OED gives for perpendicular: "At an angle of 90° to a given line, plane, or surface or to the ground", and for orthogonal: "Of or involving right angles; at right angles". In this context, I'd say either term is applicable.

Feel free to differ .

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#15
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Re: Speed Reducer Question

03/21/2017 8:42 AM

I do differ on the point.

Perpendicular lines must share a plane. Orthagonal lines need not share a plane, and while not perpendicular will be parallel to a line that is perdendicular.

Perpendicular lines intersect, orthagonal lines need not.

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#16
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Re: Speed Reducer Question

03/21/2017 9:02 AM

You misspelled orthogonal.

I do believe, sir, you would argue with a highway sign. But that is part of your charm and appeal, banana peel.

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#17
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Re: Speed Reducer Question

03/21/2017 3:11 PM

I have indeed misspelled 'orthogonal'. Thank you for the correction.

.

As to with which signs I might argue; I do not discriminate among type. I would argue any sign, be it of the highway, sober way, low way, no way, or even curds & whey type; it need only be insistently wrong.

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#19
In reply to #17

Re: Speed Reducer Question

03/22/2017 10:31 AM

RoFLMAO! Good one!

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#6

Re: Speed Reducer Question

03/20/2017 10:28 AM

120 to 1 ratio is generally a worm gear configuration....certain types of gears are used in certain types of applications....

http://machinedesign.com/whats-difference-between/what-s-difference-between-spur-helical-bevel-and-worm-gears

"Pressure angle in relation to gear teeth, also known as the angle of obliquity,[1] is the angle between the tooth face and the gear wheel tangent. It is more precisely the angle at a pitch point between the line of pressure (which is normal to the tooth surface) and the plane tangent to the pitch surface. The pressure angle gives the direction normal to the tooth profile. The pressure angle is equal to the profile angle at the standard pitch circle and can be termed the "standard" pressure angle at that point. Standard values are 14.5, 20 and 25 degrees. Earlier gears with pressure angle 14.5 were commonly used because the cosine is larger for a smaller angle, providing more power transmission and less pressure on the bearing; however, teeth with smaller pressure angles are weaker. To run gears together properly their pressure angles must be matched.[2]

The pressure angle is also the angle of the sides of the trapezoidal teeth on the corresponding rack. The force transmitted during the mating of gear teeth acts along the normal. This force has components along the pitch line and the other along the line perpendicular to the pitch line. The force along the pitch line which is responsible for power transmission is proportional to the cosine of pressure angle. The one which exerts thrust(perpendicular to the pitch line) is proportional to the sine of pressure angle. So it is advised to keep the pressure angle low. Standard pressure values are 14.5 and 20 degrees.

Just as there are three types of profile angle, there are three types of corresponding pressure angle: the transverse pressure angle, the normal pressure angle, and the axial pressure angle. [3] "

In other words the pressure angle is the angle that one gear is pushing on the other gear....

https://en.wikipedia.org/wiki/Pressure_angle

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#7

Re: Speed Reducer Question

03/20/2017 1:06 PM

I wonder what you would find if you bingled [ speed reducer ] ...

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#8

Re: Speed Reducer Question

03/20/2017 1:58 PM

#1 - we don't normally do student homework, but happy to answer questions.

#1 - since several posters are already offering to do your homework, I will chip in.

(There is no #2 on this blog - we are all #1).

You should use worm gear with tiny input shaft (having mounted upon it a tiny worm).

The bull gear (large radius gear) can be solidly mounted with a much smaller ring gear or worm gear to drive another gear as per your 90 degree set up in initial post. You should easily reach 120:1 reduction in rotation speed with 2-stage gear box, but the gears will need to be tiny. Watch for shear limits of materials to be used, as smaller teeth mean they are easier to shear off.

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#9

Re: Speed Reducer Question

03/20/2017 4:57 PM

I cant help you any better than the other guys but here's a set of gearing puzzlers I have you may find worth pondering on.

The high reduction planetary one might be of interest here.

Gearing puzzle 1. (Mismatched diameter 1:1 tooth ratio gear meshing.)

Gearing Puzzle 2. (Single stage high ratio planetary gearset.)

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#10

Re: Speed Reducer Question

03/21/2017 4:28 AM

In model-making, which is an order of magnitude smaller than the equipment suggested in the original post, the practical limit for a worm gear drive for self-propelled models is around 60:1. So a required reduction ratio of 120:1 is most likely to be in a 2- or 3-stage reduction gear-train. Reduction ratios of this nature are readily available to model-makers in the UK from specialist cottage-industry suppliers (names withheld - no endorsements).

  • In order to get the project off the ground, consider using a 2:1 spur gear driving a 60:1 worm, or, better, a 3:1 spur gear driving a 40:1 worm, and see how far it goes from there, perhaps?

Do let the forum know how the design went, for the feedback will be of value to future readers; that's how it is in Engineering.

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#20
In reply to #10

Re: Speed Reducer Question

03/28/2017 4:21 PM

Wish I had seen this the day it was due! lol

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#21
In reply to #20

Re: Speed Reducer Question

03/28/2017 4:35 PM

So not only do you expect us old farts to do your homework for you, but you brazenly expect us to deliver an answer on a deadline system? You have got to be joking.

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#24
In reply to #21

Re: Speed Reducer Question

03/30/2017 11:29 AM

No lol, I appreciate you guys going out of your way to answer

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#18

Re: Speed Reducer Question

03/21/2017 4:57 PM

So what is wrong with a planetary gear drive? Has that been considered?

https://www.youtube.com/watch?v=6X7TfOdVzpk

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#23
In reply to #18

Re: Speed Reducer Question

03/30/2017 11:28 AM

I thought it would easily reach that reduction ratio of 120:1 but the requirement that there be an input/output axis shaft that are perpendicular and intersecting disqualified that apparently

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#22

Re: Speed Reducer Question

03/30/2017 11:23 AM

Thanks for all your suggestion guys!! I should have replied sooner but its been crazy with midterms, reports, St Patricks week, St Patrick's recovery week.

At first I thought worm gear was the obvious way to go but the requirement that the output shaft and input shaft be perpendicular threw me off but I guess it was just size changes for the 2nd/3rd stage of reduction

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#25
In reply to #22

Re: Speed Reducer Question

03/30/2017 3:04 PM

Couple of extra gears (1:1) could bring the axes back into the same plane.

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