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What is pitch diameter (effective diameter) of a screw thread and what is its significance in designing a screw thread considering the strength of the thread?

umama

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What is the pitch diameter (effective diameter ) of screw thread? What is its significance in designing of a screw thread considering its "strength"?

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Umama, your question is not entirely well posed. You asked, "What is the effective diameter of the screw thread?" Well, effective in what sense? You may know what you meant, but no one else can be certain.

For a question of this sort, it is necessary to specify what is meant by "effective."

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Umama, your question is not entirely well posed. You asked, "What is the effective diameter of the screw thread?" Well, effective in what sense? You may know what you meant, but no one else can be certain.

For a question of this sort, it is necessary to specify what is meant by "effective."

​Yes, for that same purpose I have uploaded a picture with the question that labels the effective diameter. That should be self explanatory.

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This still does not really define what the effective diameter is.

You figure shows something labeled "effective diameter," but the dimensions do not terminate at any particular feature on the thread. Visually, it looks like you are saying that the effective diameter is the mean of the major and minor diameters, but that is only a rough, imprecise, visual interpretation. How could you argue with someone who said it was slightly more or slightly less than half the difference between the two? Your figure does not identify a feature that ties down the "effective diameter."

The use of a modifying adjective like "effective" implies that this diameter is "effective" or perhaps an "average" between two (or more) actual diameters. For this to make sense, it is necessary to specify the way in which "effective" is defined.

Let me give you an example. Engineer A is concerned with the strength of a bolt made with this thread. For him, the "effective" diameter is the diameter that defines the area effective in resisting breaking the bold. Engineer B is no concerned at all with the strength of the bold, but rather is concerned with coating the bolt with a lubricant. For Engineer B, the effective diameter is the diameter that defines an equivalent cylinder to be smeared with lubricant. These may, or may not, be the same diameter.

"Effective" only makes sense provided the sense in which it is used is defined.

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Yes, its true that the effective diameter is APPROXIMATELY between the major and mean diameters. And this line intersects the flank in such a way such that the width of the threads and the spaces between the threads during their engagement is same, In this case it makes a PERFECT thread.

But there are many errors induced in actual situation like pitch errors, thread angle errors, etc. whereas effective diameter is the dimension that decides the quality of fit between screw and nut.

Since engineer A is concerned with strength, he can take into account the "functional diameter" which accounts for the errors induced and is more than the effective diameter.

Since engineer B is not concerned with strength, and only intends for providing lubrication which is necessary for maintaining frictions in mating parts especially when dynamically loaded or exposed to vibrations, he/she may consider the effective diameter along with the tolerance grade specified with the pitch/effective diameters, depending on the type of engagement required.

Thus an effective diameter acts as a base for deciding the strength and tolerancing of screw threads.

 

I rephrase my posted question, earlier being  What is the effective diameter of screw thread? What is its significance in designing of a screw thread?

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Effectiveness of a machine member cannot be quantified by a mere perceptions, one should established design consideration like ( e.g. application of load , material used, fits, type of lubrication, atmospheric conditions, etc.) thats why we engineers exist its because we try to understand all these.

Engr. Umama did a right question, understanding all these thing with someone else expertise..... Its a matter of understanding.....

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