The conversion of a stepper motor from rotary motion to linear motion can be accomplished using several mechanical methods, including rack and pinion drives, belt pulley drives, and other mechanical linkages. All of these designs require various mechanical parts. However, the most efficient way to achieve this conversion is within the motor itself.
A basic stepper motor rotates by the interaction of a magnetic rotor core with a pulsating electromagnetic field generated by the stator. A linear motor converts rotational motion into linear motion; the precision of this conversion depends on the rotor's step angle and the chosen method. The linear stepper motor, also known as a linear stepper motor, first appeared in patent number 3,402,308 in 1968 , granted to William Henschke . Since then , linear stepper motors have been used in many demanding fields , including manufacturing, precision calibration, and precision fluid measurement.
The precision of a linear motor using threads depends on its thread pitch. A nut is installed at the center of the linear motor's rotor, and a corresponding screw engages with this nut. To allow axial movement of the screw, a method must be used to prevent the screw from rotating with the rotor assembly. Because the screw's rotation is constrained, the screw achieves linear motion when the rotor rotates. Typical methods for achieving rotational constraint include using a fixed threaded shaft assembly inside the motor or using a non-rotatable but axially freely movable nut on an external threaded shaft.
To simplify the design, it is meaningful to implement linear transformation within the motor. This method greatly simplifies the design, enabling precise linear movement using linear motors directly in many applications without the need for external mechanical linkages.
The earliest linear motors used a combination of a ball screw and a ball nut. The ball screw improved efficiency by more than 90%, while the trapezoidal thread provided only 20%-70% efficiency, depending on the thread conditions .
While ball screws are an efficient method for converting rotary motion into linear motion, ball nuts require precise calibration, are bulky, and expensive. Therefore, ball screws are not a practical solution in most applications.
Most equipment designers are familiar with linear motors based on hybrid stepper motors. This product has been around for many years and, like other equipment, has its own strengths and limitations. Its inherent advantages include simple and compact design, brushless operation (and therefore no sparks), remarkable mechanical benefits, practicality, and reliability. However, in some cases, this linear motor cannot be used in certain equipment because its durability cannot be guaranteed without routine maintenance.
However, several methods exist to overcome these obstacles, enabling linear motors to achieve greater durability and maintenance-free operation. Due to the brushless design of stepper motors, the only components prone to wear are the rotor bearings and the threaded engagement consisting of the lead screw and nut. Recent improvements in ball bearings have provided long-life products suitable for linear motion. The lifespan and durability of the lead screw and nut combination have also been improved recently.
Improve durability
First, it's necessary to understand the basic structure of an electric motor. A good example is the Size 17 motor, which belongs to the smaller size family of hybrid stepper motors. Conventionally, linear motors use a hollow shaft machined from a bearing-grade metal (such as bronze) with internal threads that connect to a threaded guide rod. The hollow shaft is mounted along the rotor axis. The guide rod is typically made of stainless steel, which offers considerable corrosion resistance. Most parts use machined threads (such as #10-32 ), which can be single-start or multi-start, depending on the required precision and speed of the motor.
V -shaped threads are generally chosen for machining due to their ease of processing and roll forming. While this is advantageous for machining, it is disadvantageous for power transmission. Acme threads are more suitable in comparison, primarily for the following reasons:
Acme threads are designed for greater efficiency. From a usage perspective, lower wear (including friction) translates to less wear and a longer service life. This is easily understood by examining the basic geometry of threads. The angle between the opposing faces of a " V " thread is 60 degrees, while that of an Acme thread is only 29 degrees.
Assuming the friction, torque, and thread angle are the same, the force that a " V " thread can transmit is approximately 85% of that of a trapezoidal thread . The efficiency can be calculated using equations ( 1 ) and ( 2 ) because the thread used is V -shaped and depends on the load direction. The ratio can be calculated by dividing the efficiency of a 60- degree thread by the efficiency of a 29- degree thread.
The efficiency calculations here do not take into account the additional losses caused by the high pressure on the surface of the " V " shaped thread.
Acme threaded guide rods are generally manufactured for power transmission, so their surface finish, pitch accuracy, and tolerances are strictly guaranteed. " V " type threads are basically used for fastening threads, so their surface finish and straightness are not strictly controlled.
Meanwhile, the nut driving the screw becomes even more important, as it is typically embedded in the motor rotor. Traditional nut materials use bearing-grade bronze with internal threads, a design that balances physical stability and lubrication. Of course, it's described as a balanced approach because it's not particularly excellent in either aspect. A better material for the drive nut in linear motors is a self-lubricating thermoplastic. This is because using new engineering plastics reduces the coefficient of friction in screw-nut motion. Figure 3 compares the frictional properties of different internal thread rotor materials.
The result is obvious, but why not use a plastic drive nut? Plastic is good for threads, but unfortunately, engineering plastics are not stable for the rotor journals in hybrid motors. Since the motor temperature can rise to 167 ° F during operation , the plastic can expand by up to 0.004 inches under these conditions, while brass expands only 0.001 inches under the same thermal conditions . See Figure 4 .
Bearing journals are crucial in hybrid motor structures. To achieve optimal performance, hybrid motors must be designed with a clearance of a few thousandths of an inch between the rotor core outer diameter and the stator inner diameter. If the rotor assembly is misaligned, it will rub against the stator inner wall. Designers aim to achieve good results in both thread life and bearing journal stability by selecting appropriate materials, and injection-molded metal rotor structures with internal threads are an ideal choice. (Figure 5 )
This structure significantly improves motor lifespan and efficiency while reducing operating noise. The motor lifespan is longer than that of conventional bronze nut structures used in other motors, and it requires no maintenance. (Figure 6 )
