View Full Version : Which threads are the Strongest, Acme or Square?

12-02-2005, 10:24 PM
Hi All!
I am in the process of building a 50 ton shop press. I intend to use 2 4" bore cylinders at the top with an adjustable screw (2 to 3" dia.)mounted in between them at the 2" thick base plate.
Does any one know which thread would take 50+tons of pressure?

Thanks, Hans.

12-02-2005, 10:53 PM
Hi,we have a 50 ton press at work which uses a 2-1/4" 4tpi acme thread.Almost any big pitch or profile will work ,they used the acme pitch because they wanted speed of adjustment.
The primary concern is the length of thread ingagement,on that press the length is 4" because the bore it screws into is cast iron.

Hope this helps.

New chips
12-02-2005, 10:55 PM
if the force is only going to be applyed one way use a thred that has a profile like a 30 60 90 right triangle with the right angel doing the pushing. I do not remember the name of the thred. think it was called a butres thred.


12-02-2005, 11:12 PM
A Buttress thread is the one commonly used on wood working vises. it is designed for pressure one way only and works great for rapid traverse the other. On most presses these days they use ACME. Square is strong but not easy to cut and can bind if not made properly. If I am not mistaken, ACME came out after Square as an improvement. If I remember correctly, the 50 Ton Press that I used to use had a thread size of about 2.5 inches. Someone out there will come up with a mathamatical fomula for you. My advice would be to figure it out and make it one size larger. The type of material that you will use will make a big difference also. Ther is also a thread form called "American Modified Square thread" that has an included angle of 20 degrees. As it is a 20 degree angle instead of a 29 degree(ACME) it would be stronger than an ACME. Unfortunatly I could not find it in the Machinists Handbook. I did find it in "Guide to World Screw Threads" by Industrial Press, ISBN 0-8311-1092-9.


[This message has been edited by coles-webb (edited 12-02-2005).]

Paul Alciatore
12-03-2005, 01:52 AM
I believe you will find that the difference in strenghe between square and Acme is not that great. Actually, the Acme is stronger because the thread form has a wider base. The Buttress form has the widest base possible but it's maximum strength is in one direction only while the Acme is equal in both directions.

The base of a square thread is only 50% of the pitch. The Acme has a base about 63% of the pitch and the Buttress' base is about 90 to 95% of the pitch. Also the Buttress has a triangular cross section which is exactly the form that is used for cross braceing in bridges and towers.

The main advantage of the square thread is that the force is directed parallel to the screw's axis. Thus, there is almost no tendency to compress and distort the thread like a Vee thread would suffer if under heavy stress. The modified square (10 deg sides) is a compromise that allows the thread to be cut easier while preserving much of the square's other properties.

The Acme is an effort to compromise between the square and the Vee and it is a good solution (perhaps the best) where the force must be applied in both directions, like a lead screw. The Buttress is ideal where the force will be applied in one way only. Thus it may be best for your press application. Acme would be my second choice.

Paul A.

[This message has been edited by Paul Alciatore (edited 12-03-2005).]