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thread engagement for screws 2
- Thread starter scuba
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- #2
I have used a couple of sources in determining thread engagement length. I found equations in the Machinery's Handbook and in ASME B1.1-1989 that provide formulas for determining the tensile stress area (the effective cross-sectional area) of the external thread, the shear area of the external thread, and the shear area of the internal thread. They can then be used in a provided equation for finding an initial length of engagment. After finding these results, you can determine a factor of strength (call it "J"
for the internal thread if the material is of a lower strength than the external thread. This factor is as follows:
J=[(ext. thd. shear area)x(tens. strgth. of ext. thrd.)]/[(int. thd. shear area)x(tens. strth. int. thd.)]
If J is less than or equal to 1, the length is adequate to prevent stripping. If J is greater than 1, then multiply the length of engagement by J to find the req'd. length of engagement. This will force the bolt to break first.
for the internal thread if the material is of a lower strength than the external thread. This factor is as follows: J=[(ext. thd. shear area)x(tens. strgth. of ext. thrd.)]/[(int. thd. shear area)x(tens. strth. int. thd.)]
If J is less than or equal to 1, the length is adequate to prevent stripping. If J is greater than 1, then multiply the length of engagement by J to find the req'd. length of engagement. This will force the bolt to break first.
I AGREE WITH STICK1 . I'VE USED THE MACHINERY'S HANDBOOK FOR THE ABOVE MENTIONED FORMULAS. IT DOES WORK WELL. YOU NEED TO KNOW THE MALE FASTENER MATERIAL AS WEEL AS THE FEMALE POTION BEING FASTENED TO OR THE NUT MATERIAL. IT'S A GOOD SOURCE..............PK
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One simple way to determine necessary thread engagement is to set the thread engagement at least equal to the thickness of a standard nut for the bolt to be used.
Also, if you are using aluminum plate and steel plate with steel bolts, beware of corrosion problems. Additionally, fastener seizing and galling are problems when using steel bolts in aluminum plate.
Also, if you are using aluminum plate and steel plate with steel bolts, beware of corrosion problems. Additionally, fastener seizing and galling are problems when using steel bolts in aluminum plate.
Machinery's is OK, but you tend to overdesign with it. If that's what you want go ahead.
THE source is John H. Bickford's "Introduction to the design and behaviuor of the bolted joint".
This is the definitive work on the bolted joint. It is a fantastic book. Understand what is between these pages, and you will a much welcomed engineer indeed.
THE source is John H. Bickford's "Introduction to the design and behaviuor of the bolted joint".
This is the definitive work on the bolted joint. It is a fantastic book. Understand what is between these pages, and you will a much welcomed engineer indeed.
Standard nut thickness is approximately 7/8d. Nuts are usually softer metal than the bolt to distribute clamping load more uniformly among the threads in contact. (Fundamentals of Machine Component Design 2nd ed, Juvinall & Marshek, p. 357)
I'd watch out for that metal combination, your mechanic will possibly strip the tapped hole if you don't get a longer engagement. I, too, predict galvanic reaction, though threadlock or something like that might prevent it.
I fixed my Dodge last weekend, I find Chrysler prefers to design a thread engagement more like 1.5d to 2d especially when there's only enough room for a wrench!
I'd watch out for that metal combination, your mechanic will possibly strip the tapped hole if you don't get a longer engagement. I, too, predict galvanic reaction, though threadlock or something like that might prevent it.
I fixed my Dodge last weekend, I find Chrysler prefers to design a thread engagement more like 1.5d to 2d especially when there's only enough room for a wrench!
If your tapped threads are in the aluminum, then you might want to look at thread formers such as the Taptite or PT or other tri-lobular shank fasteners. These thread-formers produce much stronger threads than a cut thread. I did this with a major automotive manufacturer on their transmission housings, went from from a drilled and tapped hole to a "as-cast" hole with the proper taper for the depth needed. You can then adjust your per cent thread engagement slightly to allow for frictional factors encountered. However, as others have cautioned you, you must use some sort of a barrier to prevent galvanic corrosion between the steel and the aluminum.
Most thread former manufacturers recommend 70% - 80% thread engagement for their thread formers in aluminum with 1.5d - 2.0d (d=screw diameter)length of engagement and will provide tables that will tell you what taper, hole size and length of engagement you will need for various size self-tapping thread forming screws.
A side benfit of using the self-threaders is that if you are using an English thread such as a #6 and you strip it out, you can sometimes move up to the metric M-4 for repair purposes, if it strips, move up to the English #8 and so forth. It is not recommended procedure but it does work, and reliably so in many cases. But, it is always best to test it out first.
You did not say what alloy aluminum you are using but sometimes in soft aluminum you can use a self thread forming tri-lobular body with wide spaced threads fastener such as the Plastite or other trade name with a dry to touch lubricant and get along fine. Bill P.
CSFT, Inc.
Most thread former manufacturers recommend 70% - 80% thread engagement for their thread formers in aluminum with 1.5d - 2.0d (d=screw diameter)length of engagement and will provide tables that will tell you what taper, hole size and length of engagement you will need for various size self-tapping thread forming screws.
A side benfit of using the self-threaders is that if you are using an English thread such as a #6 and you strip it out, you can sometimes move up to the metric M-4 for repair purposes, if it strips, move up to the English #8 and so forth. It is not recommended procedure but it does work, and reliably so in many cases. But, it is always best to test it out first.
You did not say what alloy aluminum you are using but sometimes in soft aluminum you can use a self thread forming tri-lobular body with wide spaced threads fastener such as the Plastite or other trade name with a dry to touch lubricant and get along fine. Bill P.
CSFT, Inc.
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