Square threads is used in situations in which a machine application require high precision with relatively simple task to be performed. For example, vises use square threads because they must hold an object in place without it creeping under the applied loads. Presses use square threads for the same reasons as a vises.
Additionally, manual machines use feed screw with square thread profiles because the feed screw must move a table without placing any side load on the nut that could damage it prematurely. The square thread profile is a better choice for these type of mechanical loads due to the shape of the thread profile. The name of the square thread come from the specific shape of the thread groove.
Square Threads: Uses, Advantages and Care
Both flanks of the threads stand vertical at a zero degree angle, indicating that the width of the thread is of the same size as the space between the threads. The depth of a square thread is half of it’s pitch. Because the threads are vertical in design, there is no angled force that push the nut outward.
Instead, the axial load on the nut stay within the screw, preventing wear on the nut over time. The efficiency of a square thread design allow for a machine to be more efficient in the transfer of force from the screw to the nut. Since there is no radial component of a square thread, more of the turning effort of a screw can be used to move the load rather then overcoming friction with the flanks of the threads.
The efficiency increase as the diameter of the screw increases. Although the increase in efficiency is modest, it compound over hundreds of cycles of a production machine. Many people will choose an Acme thread over a square thread because Acme threads is easier to cut.
The thirty-degree angle of an Acme thread create a radial force that the nut must fight against. Over time, this create wear on the nut and screw. Square threads do not create this wear on the nut or screw, but they are more difficult to machine proper with a tool that has vertical sides and a width that is half of the pitch of the screw.
Any side rake on a tool will ruin the thread profile of a square thread. The compound rest on a lathe must be at zero to cut the tool propery. Multi-start square threads change the behavior of the square thread without changing the profile of the thread.
Double-start threads will allow the nut to travel twice the distance of a single-start thread with the same pitch of the screw. This design feature allow for faster movement of the nut. However, it also reduce the self-locking ability of the screw.
Single-start threads are used in vises and jacks because they will not move when they are under a load. The lead angle of a single-start screw will always be less than the friction angle of the nut material. For a two-start or three-start screw, the lead angle may be more than the friction angle of the nut material, which will allow the screw to back drive under the load.
Bronze is the most common material for the nut portion of a square thread design. This is because bronze will wear down before the screw wear down. Another material that is often used for the nut portion of the square thread is cast iron.
Cast iron is used for nuts on screw designs that are used infrequently because it is less costly than bronze. Square threads must be lubricated. Way oil can be used to lubricate the screw and nut.
If way oil is applied to the threads regular, it will prevent grit from getting into the threads. This will prevent wear on the screw and nut. As the screw is used over time, the flanks of the thread will wear down.
When the backlash of the screw reaches 0.010 inches, the nut will need to be replaced. A square thread is not the ideal solution for every machine application. Ball screws are a better option for machines that require high speeds while having a near-zero backlash.
Acme threads are better than square threads in situations in which a person need to tap or die-cut a screw. However, for applications that require high axial force, high efficiency, and self-locking capability, the square thread is the best solution.
