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Power Screws - Description

The function of a power screw is to transform rotary motion into linear motion. The three types of power screws are square thread, acme thread and ball screw. Square threads and acme threads are similar in design, where the nut slides along the screw threads (like a traditional screw/nut interface). The ball screw thread uses ball bearings between the screw thread and corresponding thread on the nut. In aerospace applications, the two most common power screws are the acme thread design and ball screw design.

For a power screw with square threads, the threads follow the profile shown in Figure 1. Figure 2 shows the thread profile for an acme thread power screw. The mating nut for the screw profiles is not shown in either Figure 1 or Figure 2.

Figure 1 Square Thread Profile

Figure 2 Acme Thread Profile

In Figures 1 and 2, the distance p is the distance between the same points on adjacent teeth. p is often referred to as the screw lead, which is the distance a nut would travel for one complete rotation of the screw. The remaining dimensions of the thread are based on the dimension, p. For an acme thread screw, the thread profile is not normal to the screw centerline but is at an angle, a. The angle, a is called the thread angle. For a standard acme thread, a = 14.5. Also illustrated in Figures 1 and 2 is the definition for the major diameter, mean diameter and minor diameter of the screw. The mean diameter is often referred to as the pitch diameter. The pitch diameter is the center location of applied forces for a screw.

A ball screw thread profile is shown in Figure 3. For a ball screw, the thread profile is cut to accommodate the ball bearing. The grooves will be machined to match the ball bearing as close as possible while maintaining some clearance at the deepest part of the groove. The ball bearing contacts the thread along a contact line, with a contact angle as shown in Figure 3. For a ball screw/nut combination note that the threads between the screw and nut will be separated by a small gap (i.e., they don’t mesh together as they would for an acme thread screw/nut). Forces between the screw and nut are applied along the line of contact in the threads. Lead would also be measured at the contact point between 2 adjacent teeth.

Figure 3 Ball Screw Thread Profile

An acme thread screw is characterized by low efficiency due to the friction between the screw and nut. Because of the friction, an acme thread screw is not easily back drivable. Ball screws are characterized by having a high efficiency (due to the low friction of the balls). As such ball screws are better suited to high speed applications. Because of the low friction, ball screws are easily backdrivable. Consequently most ball screw actuators contain no back devices or motor brakes to prevent backdrive. Common applications of power screws in aerospace are flap, slat and stabilizer actuators.

A screw thread may have 1 or more starts. A single start screw will have a single thread on the screw and mating nut. A two start screw will have two threads on the screw and the mating nut. The threads will start 180 degrees apart on the end of the screw such that every other thread will be associated with 1 start. For a two start screw, the lead is measured between every other thread. For a single start screw Lead = Pitch. For a multiple start screw Pitch = Lead / number of starts. The definitions for pitch, lead and starts, and the relationships between these parameters, are the same for an acme thread screw and a ball screw.

For a ball screw, two other definitions are important:

Ball Return Paths: As a ball screw rotates, friction between the ball bearings and threads tend to push the ball bearings in one direction. A return path is thus added to all ball screws to allow the balls to cycle from 1 end of the thread back to the other end. This has the affect of reducing ball screw friction and increasing efficiency. Without a return path there may also be a tendency for a ball screw to bind. All ball screws will have a ball return path. See Figure 4 for a example of power screw ball return path.

Circuits: A circuit is the complete path for a set of balls. The circuit consists of the path between the screw and nut threads as well as the return path. For a given start within a ball screw/nut assembly, there may be 1 or more circuits. Multiple circuits are desirable for redundancy. Assuming a single circuit can carry maximum operating loads, then loss of a one circuit in a 2 circuit ball screw allows the ball screw to continue operating. Figure 4 shows an example of a ball screw circuit.

Figure 4 Ball Screw & Nut Showing Ball Return Path and Circuit

The selection of screw and nut materials is very important. In aerospace, power screws materials for the screw, nut and ball bearings (for ball screws) are normally a case hardened steel. Steel generally provides the best strength properties and it is durable from a fatigue standpoint. Case (surface hardening) of the screw and nut surface improves wear characteristics and reduces chances of galling and binding over long periods of time in service. For critical applications, such as horizontal stabilizer control, freeplay requirements are very tight. Hardening is commonly used to ensure the freeplay requirements are maintained in service. Another means used in manufacturing to maintain tight freeplay requirements is to machine the screw and nut to tighter tolerances and then use lapping techniques to open up the freeplay to the correct requirement. This approach leads to a matched screw/nut combination. In non-critical applications, which also are low load, other materials may be used. In some industrial applications, plastic screws and nuts are used. Any choice of material should be examined from a static strength, fatigue, wear and environmental standpoint. The 3 main environmental concerns are thermal affects, vibration and corrosion.

Surface or case hardening of thread surfaces are common in power screws to maximize wear characteristics, provide good surface stability and reduce backlash. Surface hardening is a material processing technique for increasing the (Rockwell) hardness of ball screw and ball nut wear surface. Surface hardening normally involves introducing carbon or nitrogen (at high temperature) into a low carbon steel thereby increasing the hardness for a thin material layer around the part. The purpose is to improve the wear characteristics of the screw and nut. The process for hardening is usually done after heat treat. Surface (case) hardening processes should be examined closely as there is a potential to induce hydrogen embrittlement in the material. It is important that the surface hardening process does not affect the heat treat and material properties of the base metal. Two methods for screw hardening are carburization (carbon is introduced into the material surface) and nitriding (nitrogen is introduced into the material surface). Other surface hardening methods exist.

During manufacture, a good practice for power screw parts is to send a material coupon through with a batch of screws and nuts during heat treat and case hardening. The coupon – blank metal about the same size of the screw or nut – can then be inspected to ensure the heat treat and surface hardening processes were done properly. Destructive inspection methods on the coupon are used to verify proper material properties for the corresponding batch of screws and nuts.

When evaluating a power screw for a given application the following parameters should be considered in the evaluation.

Lead –Lead represents the distance the nut will travel (linearly) during a single screw rotation. Pitch is defined as the distance from a point on 1 thread to the same point on adjacent tooth. Lead is also referred to as pitch for a single start screw (a screw with 2 starts will have a Lead = 2 * Pitch). Since actuators are usually specified through a linear (nut) velocity, the chosen lead (or lead angle) will have implications for motor and gear train design. For example, a motor will have a certain speed (rad/sec), which when translated through a gearbox may or may not drive the nut at the required velocity (in/sec). Hence the motor characteristics, gearing through any gearbox arrangement and the screw lead must be compatible to achieve desired overall performance. Lead may also affect accuracy. A larger lead (or lead angle) will require finer rotational position control to maintain accurate nut position control. A smaller lead angle will require a larger screw rotation to produce the same output translation. Lastly, for an ACME thread screw if the lead angle is small enough an ACME screw will be self locking (see Power Screws – Equations).

Number of Starts – Power screws may contain a single screw thread (single start thread) or multiple screw threads. For a 2 start screw, the 2 screw threads will start 180 degrees apart on the screw shaft and run intertwined along the screw. For a multiple start screw the screw lead would be measured between threads associated with a given start (for example, on a 2 start screw the lead would be measured between every other thread). Multiple start screws are often used to provide load-carrying redundancy since each start (or thread) would be sized to carry the maximum load. Also, for a ball screw this would imply that the balls could be lost for one thread and the screw would still be able to operate properly.

Number of Ball Circuits – For a ball screw/nut, there may be 1 or 2 ball circuits. Two ball circuits would provide redundancy if each circuit is designed to carry ball loads. In this case, if one circuit should fail such that all of the balls were lost, the other circuit would maintain operation of the ball screw. Normally, failure of a ball circuit is considered a latent failure. In aerospace, it is unlikely that a two start screw would have two circuits for per start (4 ball circuits total) as this amount of redundancy is not generally required. However, for a single start screw the use of a 2 ball circuit design should be evaluated from a redundancy perspective.

Return Path Design Characteristics – The design of the ball return path for each circuit should be carefully evaluated. The circuit should be designed to withstand the repetitive impact loads of the balls flowing through the circuit. In addition, the ball circuit “cover” or support should have sufficient fasteners such that the loss of one fastener will not degrade operation of the ball circuit. Fasteners should have appropriate locking features to prevent the fasteners from backing out. A tube mounted on the side of the nut can be a problematic design as it can be difficult to provide proper structural support for the tube. A plate with machined ball path is generally a more robust design.

Load, Limit (or Max Operating) – Limit load is the maximum load the power screw must be able to move. The static load is used to size the screw threads and is also used to evaluate column load characteristics of the screw. Limit load is used in static load analysis. Ultimate load will be 1.5 times limit load. In some cases, specifications will give a limit load that is above max operating load by a certain factor (10-25% higher). Therefore, it is important to ensure limit load, maximum operating load and ultimate load are clearly defined when specifying or analyzing a power screw.

Endurance and Fatigue Loads – Endurance and fatigue loads represent the normal operating loads that a power screw would be expected to see over its operating life. Endurance and fatigue loads are usually the same spectrum. Endurance is associated with the power screw operating (rotating) and fatigue loads are associated with the power screw held statically. Like static loads, fatigue loads are used to size the screw through fatigue analysis. Power screws with high load and low cycles will likely be sized by the static load while power screws with requirements for high operational cycles will likely be sized by endurance loads. Endurance loads are normally associated with cycles (i.e., so many cycles at one load, so many cycles at another load and so on). Loads may also vary with a function of nut position on the screw. Endurance tests are normally done with the power screw (or actuator) operating and fatigue loads are applied with the power screw static (not rotating). Fatigue requirements will specify the number of lifes that a screw must be tested. For example, if 1 life of endurance and 4 lives of fatigue testing are required a common practice is to do 1 life of endurance (power screw operating with endurance load spectrum applied) and 3 lives with the endurance/fatigue load applied with the power screw not operating.

Side Load – A power screw application may have a side load imparted to the screw. Side loads should be eliminated or kept to an absolute minimum. Side loads will impact the life of the screw and can also lead to a larger screw to account for column loading in combination with side loads. Sides loads can be minimized by proper actuator installation. Using spherical bearings for attach points will keep side loads minimal.

Screw Material – Screw material should be chosen carefully. Stainless steel is the usual choice aerospace power screws. The material is heat treated to achieve a desired hardness range (Rc 30 and above). In some cases for acme thread screws, the Rockwell hardness between screw and nut will be different. For power screws used in critical flight components, the material and process specifications should be clearly specified and the manufacturing process tightly controlled. In some cases, it may be necessary to specify material properties tighter than the general material or heat treat specification.

Surface (Case) Hardening – Surface hardening will likely be required for critical power screw applications. The 2 most important aspects are to ensure proper specifications are in place to define the surface hardening process and that proper checks are done during manufacture to ensure the surface hardening process only hardens the outermost surface (usually around a few thousands of an inch) and not the complete part. Sending test coupons through the hardening process with a each batch of screws and nuts is a good method to allow inspection of the process without having to destroy an expensive machined part. A qualified metallurgist should review all material process specifications.

Surface Finish – For power screws to work at highest efficiency, the running surface finish should be as smooth as possible. Surface plating of power screws surfaces is not a good idea because of the potential for flaking of the finish, which will lead to increased and accelerated wear on the surfaces.

Temperature – In aerospace applications, power screws operate over a wide temperature range. Material properties can change over the full range. In addition, thermal expansion and contraction may lead to binding during operation. Both of these aspects should be validated in a power screw application through testing.

Balls (Ball Screw) – The balls in a ball screw are generally hardened steel ball bearings manufactured to ball bearing specifications. The controlling specifications for the ball bearing should be reviewed for compatibility with the load and life expectancy for the balls. Additionally, each circuit will have a fixed number of balls such that the balls completely fill up the circuit with minimal freeplay in the circuit.

Speed Rating (Ball Screw) – Speed rating generally applies to a ball screw configuration. A acme thread power screw is not suitable to long, high speed duty cycles. For a given ball screw design, there will be a critical velocity of the balls. Above a certain speed a resonance vibration will occur in the power screw assembly. The vibrations result from the repetitive force of the balls hitting structure in the return path and hitting each other as they flow through the ball circuit.

Backlash (Freeplay) – A power screw will always contain a specification for backlash or freeplay. Backlash or freeplay is determined by applying a small load in one direction and then measuring the distance the nut moves when applying the same load in the opposite direction. In measuring freeplay, the load should be small enough so that structural deflections are not occurring (a few percent of maximum load is usually sufficient). Freeplay can lead to flutter when a power screw is connected to a flight surface. Also, freeplay is a measure of control accuracy. Anti-backlash nuts can be used to reduce freeplay. However, use of anti-backlash nuts on horizontal stabilizer and flap actuators is not used in practice.

Lubrication – Most power screws are lubricated and require periodic lubrication when in service. Both the type of lubrication (normally grease) should be examined for compatibility with the materials. Periodic lubrication intervals should be established on the basis of test or service experience with a similar power screw application. Endurance testing is good approach for validating a chosen grease and a lubrication interval.

Life – Power screw are designed for a certain life at a given load. Life is generally given in terms of inches of travel at a given load. For a power screw designed to a new specification, the power screw will be designed for desired load and life. If an off the shelf power screw is used, the life rating will need to be adjusted for any application.

Efficiency – Efficiency affects the choice of a acme thread or ball screw. When a high efficiency power screw is required, a ball screw actuator will be required. Efficiency of the power screw will affect the design and sizing of the power drive (either electric or hydraulic motor).

Left Hand or Right Hand Thread – The thread orientation determines which direction extends the nut on the screw and which direction retracts the nut on the screw. This will affect the drive gear box design and motor polarity so the proper command signals match extend and retract directions of the nut.

Other items associated with power screw applications include no backs devices, load path redundancy, seals, scrapers, drive torque and gearbox interface. These items are covered in the electromechanical actuator module (see Actuator, Electromechanical – Description).

Qualification testing for power screws should include all of the mechanical environmental tests required by RTCA/DO-160 or Mil-Std-810 or other appropriate environmental specification. Normally environmental tests will be done at the actuator level, which will include the power screw, gearbox, any no-back device and any electronics installed in the actuator.