Lead Screw Mechanism – types , Advantages
Lead Screw Mechanism – Lead screw Examples
What Is Lead Screw ?
A lead screw, also known as a power screw, is a threaded rod or bar that translates rotational motion into linear motion. When it comes to translating the rotary motion of a motor to linear motion, there are many different ways of accomplishing this task. Often times in the world of motion control, the battle between the different methods and technologies can get quite contentious. However, as a rule, it’s best to remember that specific applications are better for some technologies than others, and as always, cost and product life time are also critical factors in determining which technology to use in a given application.
Lead screws use the helix angle of the thread to convert rotary motion to linear motion.The performance of a lead screw is heavily dependent on the coefficient of friction between the nut and the screw, which in turn depends upon the material used for the nut and screw. Lead screws typically use nuts made of internally lubricated plastic or bearing-grade bronze. Plastic nuts usually travel on stainless steel screws while bronze nuts often run on carbon steel screws. When bearing-grade bronze nuts are used, stainless steel screws are an option.
Leadscrews are manufactured in the same way as other thread forms (they may be rolled, cut, or ground).
A lead screw is sometimes used with a split nut also called half nut which allows the nut to be disengaged from the threads and moved axially, independently of the screw’s rotation, when needed (such as in single-point threading on a manual lathe).
Types Of Lead Screws are:
Power screws are classified by the geometry of their thread. V-threads are less suitable for leadscrews than others such as ACME because they have more friction between the threads. Their threads are designed to induce this friction to keep the fastener from loosening. Leadscrews, on the other hand, are designed to minimize friction. Therefore, in most commercial and industrial use, V-threads are avoided for leadscrew use. Nevertheless, V-threads are sometimes successfully used as leadscrews, for example on microlathes and micromills.
Square threads are named after their square geometry. They are the most efficient, having the least friction, so they are often used for screws that carry high power. But they are also the most difficult to machine, and are thus the most expensive.
Acme threads have a 29° thread angle, which is easier to machine than square threads. They are not as efficient as square threads, due to the increased friction induced by the thread angle. ACME Threads are generally also stronger than square threads due to their trapezoidal thread profile, which provides greater load-bearing capabilities.
Buttress threads are of a triangular shape. These are used where the load force on the screw is only applied in one direction. They are as efficient as square threads in these applications, but are easier to manufacture.
Advantages & disadvantages
The advantages of a leadscrew are:
Large load carrying capability
Simple to design
Easy to manufacture; no specialized machinery is required
Large mechanical advantage
Precise and accurate linear motion
Smooth, quiet, and low maintenance
Minimal number of parts
Most are self-locking
The disadvantages are that most are not very efficient. Due to the low efficiency they cannot be used in continuous power transmission applications. They also have a high degree of friction on the threads, which can wear the threads out quickly. For square threads, the nut must be replaced; for trapezoidal threads, a split nut may be used to compensate for the wear