Power Screw Torque Calculator
Estimate raising torque, lowering torque, collar drag, efficiency, and self-locking behavior for jacks, vises, clamps, and shop press screws.
Choose a common power screw setup. Each preset fills the load, mean diameter, lead angle, thread friction, collar radius, and collar friction fields.
Power Screw Torque Results
Calculation breakdown
| Contact condition | Thread mu | Collar mu | Calculator note |
|---|---|---|---|
| Clean greased steel on bronze | 0.08-0.12 | 0.06-0.10 | Common jack and vise assumption |
| Light oil steel on steel | 0.12-0.18 | 0.10-0.16 | Use middle value for shop checks |
| Dry steel on steel | 0.18-0.30 | 0.16-0.25 | Torque rises quickly when dry |
| Needle or ball thrust bearing | same screw | 0.01-0.04 | Collar torque can nearly vanish |
| Dirty or damaged threads | 0.25-0.40 | 0.20-0.35 | Inspect before relying on result |
| Thread form | Half-angle | Friction adjustment | Typical use |
|---|---|---|---|
| Square power thread | 0° | mu unchanged | High efficiency lifting screws |
| Acme thread | 14.5° | mu x sec 14.5° | Vise, clamp, and jack screws |
| Metric trapezoidal | 15° | mu x sec 15° | Metric presses and actuators |
| 60 degree V thread | 30° | mu x sec 30° | Fasteners, light adjusters only |
| Preset | Load range | Mean diameter | Lead or angle |
|---|---|---|---|
| Bench vise Acme screw | 1500-3000 lbf | 5/8-7/8 in | 0.16-0.25 in lead |
| Car scissor jack screw | 3000-5000 lbf | 3/4-1 in | 3-5 degree angle |
| Shop press screw | 10-40 kN | 22-36 mm | 4-7 mm lead |
| Machine leveling jack | 5000-15000 lbf | 1-1.5 in | 0.18-0.25 in lead |
| Fine adjust lab jack | 100-500 lbf | 1/4-1/2 in | 0.03-0.08 in lead |
| Check | Likely self-locking | Likely backdrive | Design note |
|---|---|---|---|
| Thread only | mu effective > tan lead angle | mu effective < tan lead angle | Collar friction not included here |
| Total screw and collar | Lowering torque is positive | Lowering torque is negative | Positive means torque is needed to lower |
| High efficiency screw | Usually not self-locking | Common with ball or steep lead | Needs brake or holding device |
| Clamp or vise screw | Usually self-locking | Rare unless heavily lubricated | Recheck after lubrication changes |
Power screws is a type of mechanical component that can be found in a variety of different machines. Power screws are used to move a load along an axis of a threaded shaft. A variety of machines that contain power screws include bench vise, bottle jacks, shop presses, and lab tables that require adjustments to there height.
When a person turns the threaded shaft of a power screw, the resulting torque has to overcome the friction create by the threads of the screw, as well as the friction created by the collar that rests upon the screw. The designer of the machine calculates the amount of torque that must be applied to the screw to ensure that the screw does not experience too little torque to stall, or too much torque which can damage the screw or the operator. The threads of the screw determine how much friction acts upon the screw.
Friction and Torque in Power Screws
Screws with square threads experience the lowest amount of friction. Other types of threads, such as those with an Acme or trapezoidal shape includes a slight angle to the threads to make them easier to manufacture, but that angle also increases the friction of the threads. Thus, any change in the angle of the threads impact the friction between the threads, which changes the amount of torque that is required to turn the screw.
Another factor that impacts the friction of a power screw is the friction created by the collar that surround the screw. The friction created by a plain thrust collar can account for thirty or forty percent of total torque created by a heavily loaded power screw. This percentage increase if the radius of the thrust collar is increased or if the thrust collar is dry.
One way of reducing the friction created by the collar is the use of a needle bearing or ball thrust bearing. However, using a bearing reduce the natural friction that prevents the screw from being back-driven by the load. A screw is said to be self-locking if the friction of the threads is strong enough to ensure that the screw will remain in place after the load is released from turning the screw.
If the friction of the threads are not enough to provide for this self-locking function, the load may tend to move the screw against its threaded axis. Many applications that use power screws require the screw to be self-locking so that the load will remain in place after the screw handle is release. In contrast, other lifting applications may require a screw that is not self-locking, so that the load can be lowered in a controlled manner.
Another factor that can influence the friction that is created by a power screw is the lubrication of the screw. A light film of oil can reduce the friction between the threads of a screw by half relative to the friction created between dry steel threads. Similarly, the addition of oil can also reduce the friction between the collar and screw.
For instance, a vise that is stiff to turn when the handle is manipulated may begin to turn more easy after oil is added to the threads. The opposite is also true for dirty screws with dried lubricant or damaged threads. Increased friction created by these issue can significantly increase the torque that is required to move the screw.
Thus, regular inspections of power screws can help to ensure that the increased friction between the threads or between the screw and the collar is not too great to significantly increase the effort that is required of the person turning the screw. In addition to friction, the other factor that can impact the power screw is the rate at which the screw turns. While the rate at which the screw turns will not impact the torque that is calculated for the screw, a screw that reaches high rates of revolutions per minute will require more horsepower to turn the screw at that high speed.
Additionally, the higher the speed at which the screw is turned, the higher the travel rate of the load that is being move by the screw. Such high speeds can make it more difficult to control back-driving of the screw. Thus, the desired speed for fine adjustment screws is usually low.
In determining the amount of torque that is required by a power screw, there are a variety of variables that the screw designer considers. For instance, variables for the calculation may include the load that is to be moved by the screw, the mean diameter of the screw, the lead of the screw, the friction that occurs between the threads of the screw, the radius of the thrust collar of the screw, and the friction between the screw and the thrust collar. The type of thread that is used will impact the friction coefficient that is used in the calculation.
Additionally, the lead-angle source of the screw can be entered as the lead of the screw or the angle of the lead. Finally, the designer can adjust the safety factor for the screw so that the torque that is calculated for the raising of the load is scaled to an amount that is comfortable for the user to turn with their handle. A common error in the calculation of the torque requirements for a power screw may be in the treatment of friction as a fixed number.
For instance, if a design calculates the friction of the threads assuming a friction between clean steel threads and bronze pulleys, but the screw experiences rusting or dry condition, the torque will be calculated to be too low to account for the additional friction that develops between those rusted or dry threads. Another common error may be the ignoring of the friction between the screw and its thrust collar. Thus, friction between the screw and its collar will lead to an underestimation of the required torque to turn the screw.
Thus, the accuracy of the calculation of the required torque is dependent upon the inputs for the friction of the screw and the load of the screw. In addition to the variables that is calculated for the determination of the required torque for a power screw, there are a variety of other factors that can impact the performance of a power screw in the real world. For instance, the effect of changes in temperature on the screw will change the viscosity of the lubricant that is contained within the screw’s threads, as well as the clearance between the threads.
Similarly, if the screw is subjected to shock load that are significantly higher than the load that is calculated for the screw, the screw may be damaged. Finally, other factor related to the operator may impact the performance of the screw. For instance, the length of the handle that is used to turn the screw, and the comfort of the operator in gripping that handle will impact the torque that can be applied to the screw.
Each of these factor is important in determining whether or not the calculated torque is a practical amount to be applied to the screw on the shop floor.
