Design of a Gear Mesh

Design of a Gear Mesh

A useful decision set for spur and helical gears includes

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Function: load, speed, reliability, life, Ko

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Unquantifiable risk: design factor nd

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Tooth system: φ, ψ, addendum, dedendum, root fillet radius

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Gear ratio mG, Np, NG

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Quality number Qv

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Diametral pitch Pd

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Face width F

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Pinion material, core hardness, case hardness

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The first item to notice is the dimensionality of the decision set. There are four design

decision categories, eight different decisions if you count them separately. This is a larger

number than we have encountered before. It is important to use a design strategy that is

convenient in either longhand execution or computer implementation. The design decisions

have been placed in order of importance (impact on the amount of work to be redone in

iterations). The steps are, after the a priori decisions have been made,

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Choose a diametral pitch.

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satisfactory, return to pitch decision for change.

Examine implications on face width, pitch diameters, and material properties. If not

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satisfactory, return to pitch decision and iterate until no decisions are changed.

Choose a pinion material and examine core and case hardness requirements. If not

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satisfactory, return to pitch decision and iterate until no decisions are changed.

With these plan steps in mind, we can consider them in more detail.

First select a trial diametral pitch.

Choose a gear material and examine core and case hardness requirements. If not

Pinion bending:

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Select a median face width for this pitch, 4π/P

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Find the range of necessary ultimate strengths

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Choose a material and a core hardness

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Find face width to meet factor of safety in bending

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Choose face width

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Check factor of safety in bending

Gear bending:

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Find necessary companion core hardness

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Choose a material and core hardness

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Check factor of safety in bending

Pinion wear:

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Find necessary Sc and attendant case hardness

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Choose a case hardness

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Check factor of safety in wear

Gear wear:

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Find companion case hardness

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Choose a case hardness

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Completing this set of steps will yield a satisfactory design. Additional designs

with diametral pitches adjacent to the first satisfactory design will produce several

among which to choose. A figure of merit is necessary in order to choose the best.

Unfortunately, a figure of merit in gear design is complex in an academic environment

because material and processing cost vary. The possibility of using a process depends

on the manufacturing facility if gears are made in house.

After examining Ex. 14–4 and Ex. 14–5 and seeing the wide range of factors of

safety, one might entertain the notion of setting all factors of safety equal.

gears, wear is usually controlling and

The use of softer cores can bring down

them higher. A tooth broken by bending fatigue not only can destroy the gear set,

but can bend shafts, damage bearings, and produce inertial stresses up- and downstream

in the power train, causing damage elsewhere if the gear box locks.

Check factor of safety in wear9 In steel (SH)P and (SH)G can be brought close to equality.(SF )P and (SF )G, but there is value in keepingGear material, core hardness, case hardness