Lead Screw | FAQs

Generally speaking, any threaded rod that is used to create linear motion could be considered a leadscrew… However… most people assume some level of accuracy and precision when they use the term “leadscrew”, making this a more precise component than a threaded rod you could buy at a hardware store.

When people talk about ACME screws they should be referring to a leadscrew with a specific type of thread profile on the screw. This screw profile is called an ACME thread… Many people use the term ACME screw synonymously with leadscrew but that’s not exactly correct. All ACME screws are leadscrews, but not all leadscrews are ACME screws. It depends on the details of what the thread on the screw looks like.

Not really, the ACME thread profile was developed originally way back in 1894. It became an industrial standard over 100 years ago. When this thread profile was developed it represented the height of technology (… in 1894). Things have changed since then, and many companies have developed better thread profiles with higher efficiency and smoother motion.

Nuts can be made of many different materials. Many years ago, the most common material was some sort of Bronze with a lubricant. This has changed to the use of polymer nuts that are often self-lubricating and eliminate the need for lubrication and maintenance.

A range of polymers can be used with different properties such as coefficient of friction, maximum pressure rating, maximum velocity rating, and the product of pressure x velocity called “PV”. Other factors to consider are temperature rating, thermal expansion, chemical resistance and more.

Backlash describes the possible free movement of the nut relative to the screw without rotation. This is caused by the clearance between the outside threads of the screw and the inside threads of the nut.

Backlash is not necessarily bad, depending on your application. If you are doing very rough positioning, such as trying to open or close a door, then a few thousands of backlash doesn’t have any negative impact. In fact, the clearance that creates the backlash also lowers the drag force in the system making it more efficient. On the other hand, if you’re trying to do very accurate “bi-directional” positioning such as a pick and place machine, then backlash could create problems with positional accuracy. Most manufactures of leadscrews and nuts have developed some way to address the problem. This usually involves some sort of spring that takes out free movement between the screw and the nut.

We’ve been using leadscrews in our actuator products for decades and we’ve seen the good , the bad and the ugly. When we got into the business of making world-class leadscrews, we wanted to address the problems we’ve seen with other anti-backlash systems. Our approach is to use a constant force spring to provide a radial force on the nut to take up all the clearances between the screw and nut without relying on axial springs. We call this patented design CFT… Constant Force Technology. It’s quite simply a better mousetrap. It provides much better control over pre-load and is dramatically more consistent over the life of the product.

You will see no backlash at any load, meaning there will be no free movement. However, if there is a spring in the system and you apply enough load you could start deflecting the spring. When this happens, you will see relative movement between the screw and nut. This could be thought of more as deflection rather than backlash, but it has the same effect – the nut is moving axially relative to the screw without any rotation.

There are few sources that tend to be the largest contributors to system error. The first is lead-error. This is the difference in the amount of linear movement actually seen versus the ideal. Example – you have a 2mm lead so you expect that one revolution of the screw will cause the nut to go exactly 2.000 mm. If it goes 2.2 mm instead of 2.0 mm then you have 10% lead error (and a pretty bad leadscrew!). This type of error can be seen in one revolution, or as an accumulating error of multiple revolutions. Another source of error is caused by leadscrew straightness. If the screw is not straight, then the geometry of the system could result in linear position errors. In addition to these concerns about the leadscrew itself, system level issues such as concentricity of the bearing system, mechanical coupling, runout, motor position errors and more could all contribute to linear position errors.

The standard operating temperature of PBC LINEAR Linear lead screw nuts is -40C~+120C