An NPT (National Pipe Thread) internal thread is a type of tapered thread used for joining pipes and fittings together in plumbing and other mechanical systems. It is commonly used in North America and is known for its leak-resistant properties.

Below is a sample chart showing the relationship between lead angle and screw efficiency, for both ball screws and lead (sliding) screws. As you can see, ball screw efficiency begins to reach its maximum at a lead angle of 10 degrees.

The terms “lead angle” and “helix angle” are often used interchangeably, but the helix angle is the angle between the helix of the thread and a line parallel to (rather than perpendicular to) the axis of rotation. Looking again at the depiction of lead and helix angles, you can see that they are complementary; their sum must equal 90 degrees.

Yes, there are alternatives to machining a NPT internal thread, such as using a threaded insert or using a compression fitting. However, these alternatives may not be as strong or as leak-resistant as a properly machined NPT internal thread. It is important to consider the specific requirements and specifications of the project before deciding on an alternative method.

The helical nature of a screw thread can be compared to an inclined plane wrapped around a cylinder, like a rotary wedge.

Filed Under: Ball + lead + roller screws, Linear drives (all) Tagged With: helix angle, lead angle, screw geometry, screw thread, thread angle

The thread design of a power transmission screw—whether a lead screw or a ball screw—plays a critical role in the screw’s function. Aside from the basic thread form (acme, trapezoidal, etc.), the three main aspects of the thread geometry—helix angle, lead angle, and thread angle—help to distinguish different types of screws from each other and play a role in determining their performance characteristics.

The first step is to prepare the material by cleaning and smoothing the surface. Then, using the appropriate tap and tap wrench, cut the threads into the material at a slight angle. Next, use the reamer to smooth and finish the threads. Finally, use a thread gauge to check the accuracy of the threads and make any necessary adjustments.

For ball screws, the lead angle also influences the preload torque of the screw assembly, which is used in drive torque calculations.

The efficiency of a screw is mainly influenced by three factors: friction, lead, and lead angle. For lead screws, the high friction produced by sliding contact between the screw and the nut means that friction is the most significant contributor to efficiency. Conversely, since ball screws use rolling elements with very little friction, their efficiency is primarily dependent on lead angle.

Thread angle is a common characterization of lead screws, with acme screws having a thread angle of 29 degrees and trapezoidal screws having a 30 degree thread angle. Ball screws, on the other hand, use a radial raceway that conforms to the rolling ball elements, so they are typically characterized by their raceway geometry—Gothic arch or circular arc.

Some common mistakes to avoid when machining a NPT internal thread include not using the correct size tap, using too much force when cutting the threads, and not properly aligning the tap with the material. It is also important to use cutting fluid and to periodically clean the tap to prevent buildup and ensure smooth cutting.

The tools needed to machine a NPT internal thread include a tap, a tap wrench, a reamer, and a thread gauge. The tap is used to cut the threads into the material, the tap wrench holds the tap in place, the reamer is used to smooth and finish the threads, and the thread gauge is used to check the accuracy of the threads.

Recalling from trigonometry class, the tangent of an angle equals the side opposite the angle divided by the side adjacent to the angle. So the tangent of the lead angle, B, equals the lead of the screw divided by the circumference of the screw.

Copyright © 2024 · WTWH Media LLC and its licensors. All rights reserved. The material on this site may not be reproduced, distributed, transmitted, cached or otherwise used, except with the prior written permission of WTWH Media.