Initial estimation of diametral pitch in gears 1

[thought2007]

Hi,

How to estimate diametral pitch before starting Spur gear design,
In usual practice i use to calclulate by iterative process ,first finding trasmitting force,than comparing with allowable force from the material with fixed face width. i feel its lengthy process.
Is there any mathematical relation to arrive diametral pitch.
One more question how to decide number of stages in gear train i guess by considering size,stages will be decided any other ways ,other than this

Thanks in before

John

 

[Philrock]

There are formulas in Machinery’s Handbook for doing preliminary estimates for gear sizing. There is also software for doing this. Probably the best is GearCalc by GEARTECH.

AGMA 901 gives methods for preliminary sizing gears for minimum total weight.

In selecting the number of stages, there are a number of rules of thumb:

1. Avoid ratios of more than 8:1 in a single stage. (This rule is widely violated.)

2. Avoid small numbers of teeth in the pinion. Really small numbers of teeth will result in undercut. However, even with numbers just large enough to avoid undercut, the number may be small enough to promote scoring. Rule of thumb for heavily loaded power gearing is 25 or more teeth in the pinion. If the gears are lightly loaded, it’s o.k. to go down to the minimum number of teeth to avoid undercut, but be sure to check for scoring, durability, and strength.

Typical applications favor compactness. Toward this end, the large gears in the various stages are often similar in diameter, resulting in the higher speed (finer pitch) stages of a reducer tending to have higher ratios than the low speed stages.

 

[gearcutter]

Here is as link to a basic online calculator:

http://wwweng.uwyo.edu/commend/Sgdc/.

You'll need to have Java installed on your computer. You can download it free from here:

http://jdl.sun.com/webapps/download/AutoDL?BundleId=10778

 

[plasgears]

Put yourself in the learn mode, and run iterative computer runs using one of the popular gear programs. Try to set limits on face width and diameter in the first stage. You will soon learn the pitch that makes sense for your envelope, knowing what level of stress is acceptable.

Remember that as you go up the gear train, the pitch should be courser and the face width wider. The gear diameter should be allowed to increase progressively, also. Practical limits on size will dictate when you need to go from plastic to metal in the upper stages. Here is a possible scenario based on moderate first stage torque:

Stage pitch diameter face width
1 48 1" 0.125"
2 32 1.2 0.187
3 24 1.4 0.250
4 16 1.6 0.312, for example.

Endeavor to limit the pinion tooth count to 11 min in each stage. This is for stacked gears. If your envelope requires nested gears (pinions alternately up and down) then the pinion count needs to increase about 5 every stage for the same pitch in succeeding stages. Your design could have both stacked and nested gears. Incidentally, you will have to progressively increase gear pin diameter, too. Have fun.

 

[israelkk]

There is no simple best diametral pitch. If you will analyse a given size gear and pinion (given pitch diameters and widths) you may find that smaller teeth will be better for surface pitting and scoring than larger teeth.

Because for the same size gear there will be more small teeth than large teeth. More teeth give larger contact ratio and smaller specific sliding. Add to this that large tooth has larger stresses at the root (longer beam with same force). Therefore, there is a point where you have to decide where to stop decreasing the tooth size. This is where there is a balance between bending fatigue life cycle and surface fatigue life cycle.