Why does high contact ratio in gear result in inefficiency?

[hihiip201]

As titled.This was taught to me in machine design, but I don't see how that makes sense

I would think that doesn't matter how many teeths are in contact, since the force between gears are distributed on each pair of teeth in contact, more engaging teeth just mean the friction is distributed to more teeth pairs(as the normal force is distributed to more teeth pairs).thanks

 

[Baluncore]

There is no real need to have more than one tooth in contact except during the transfer from each tooth to the next.

To have more teeth in contact requires smaller teeth on the same diameter gear wheel, or the same sized teeth on a bigger wheel.

Teeth should be designed to carry the entire load. Smaller teeth are a riskier proposition since wear and misalignment can unload one tooth and transfer all the load to another.

So multiple teeth in contact requires bigger gears which are heavier, they need to be more accurate and so cost more.

 

[hihiip201]

There is no real need to have more than one tooth in contact except during the transfer from each tooth to the next.

To have more teeth in contact requires smaller teeth on the same diameter gear wheel, or the same sized teeth on a bigger wheel.

Teeth should be designed to carry the entire load. Smaller teeth are a riskier proposition since wear and misalignment can unload one tooth and transfer all the load to another.

So multiple teeth in contact requires bigger gears which are heavier, they need to be more accurate and so cost more.

so are smaller teeths more vulnerable to friction and wear? => less efficiency?

or it's because they are heavier hence need more rotational kinetic energy when transmitting power?

 

[Baluncore]

why does high contact ratio in gear result in inefficiency?
This was taught to me in machine design, but I don't see how that makes sense

Unfortunately, the precise context and the meaning of the word “efficiency” have not been recorded.

The answer to your question is certainly not simple. It is multi-factorial and so requires many interrelated effects to be considered. The line between theory and practice is determined by how accurately the gears can be manufactured and supported.

It is good practice to maintain a contact ratio of 1.2 or greater. Under no circumstances should the ratio drop below 1.1, calculated for all tolerances at their worst-case values. A contact ratio between 1 and 2 means that part of the time two pairs of teeth are in contact and during the remaining time one pair is in contact. A ratio between 2 and 3 means 2 or 3 pairs of teeth are always in contact. Such as high contact ratio generally is not obtained with external spur gears, but can be developed in the meshing of an internal and external spur gear pair or specially designed non-standard external spur gears.

Are you talking spur, helical, internal or external gears ?

Indeed, a high contact ratio can lead to greater efficiency, if you are prepared to pay for the increased mass and machining accuracy, as is evident from this extract.

This advantage of having several sets of engaging gear teeth in contact at the one time, distributing the total tooth load over a number of pairs of meshing teeth, places a high premium upon accuracy in tooth form and precision in tooth spacing, for, not only does the number of teeth in contact influence the questions of required tooth strength, pitch, and durability but also the speed at which the gearing can be safely operated. Shock, noise, vibration, and excessive gear-tooth wear result when there are even relatively minor irregularities in tooth proportions and spacings, or slight imperfections in profile finish. The actual strength of the individual gear teeth may even become of secondary importance, the major points, provided the gear teeth are well formed, having to do more with tooth wear, proper proportions of gear diameters, hardness of gear teeth, and the best combination of hardness for wear.
GEAR EFFICIENCY.
Under ordinary working conditions, the frictional losses between the teeth of engaging, high-quality, cut gears and pinions should not exceed more than 1 or 2 per cent of the power transmitted. This quite moderate loss is influenced more by the length of the tooth addendum (increasing slippage) than by the obliquity of the gear teeth, and the degree of efficiency attained is, for all practical purposes, independent of the load transmitted by the gear assemblage. Furthermore, the differences in efficiency of the several standard tooth forms, or systems of gearing, are really so small as to exercise little or no controlling influence on the particular tooth form to be recommended for any specific service.
In general, as the efficiency of gearing depends for the most part upon the uniformity of the angular velocity of the pitch surfaces of the engaging gears, it follows that the finer the pitch of the gears and the more numerous the teeth, the more uniform is apt to be the angular velocity of the engaging pitch surfaces and the higher the operating efficiency of the gearing. For a given pitch, the efficiency of the gearing tends to increase with the number of teeth, so that large gears, if properly mounted, balanced, and supported, are relatively more efficient than smaller gears. This improvement is because the inaccuracies in profile finish, form, and tooth spacings, the chief causes of variations in angular velocity of the pitch surfaces, become relatively less disturbing as the number of teeth increases.

The efficiency of power transmission is one thing. The efficiency of the machine as a whole is another. These days, minimum material and shipping weight is the primary requirement. This applies to the gear and it's mountings.

Because the teeth will not share the work fairly under all conditions, every tooth must now be capable of carrying the full load. Therefore the tooth size cannot be reduced. In order to have multiple teeth in contact requires a longer contact zone and so the gear radius must be linearly greater. Mass and cost will rise as to the square of the radius.

So to have a greater contact ratio you must have more accurate tooth profiles, with bigger gears in a more rigid mounting.

But what do you gain from such a high contact ratio? You may change your friction loss from 1.5% to 1.4% but the total weight of your machine may rise by 10%. At the machine level that may be very inefficient both in terms of energy and economy.