Power Screw.xls



Purpose of calculation
: For an applied load on a screw thread calculate the axial thrust of a nut. Calculate bolt pretension from screw parameters (thread pitch, thread coefficient of friction, head coefficient of friction etc.).

To perform power screw calculations, the following steps can be taken:

a) Determine Screw Parameters:

  1. Screw Pitch (P): The distance between adjacent threads on the screw. It can be calculated by dividing the axial movement per revolution by the number of threads.
  2. Lead (L): The axial movement of the screw per revolution. It is equal to the pitch multiplied by the number of threads.
  3. Thread Depth (h): The distance from the crest to the root of the thread. It depends on the screw design and can be obtained from standard screw design tables.
  4. Mean Pitch Diameter (D): The average diameter of the screw thread. It can be calculated by subtracting twice the thread depth from the outer diameter of the screw.
  5. Helix Angle (?): The angle formed between the helix of the screw thread and the axis of the screw. It can be calculated using trigonometric functions based on the lead and mean pitch diameter.

b) Estimate Starting Torque:

  1. Determine the Running Friction Torque (Tr): Calculate the torque required to overcome the running friction using the known diameter, running friction coefficient, and axial load.
  2. Estimate the Starting Friction Torque (Ts): Multiply the running friction torque by a factor of 1.33 to account for the higher starting friction.
  3. Add the Starting and Running Friction Torques to obtain the total Starting Torque (Tstart).

It is important to note that power screw calculations can involve additional factors and considerations, such as efficiency, lubrication, and system dynamics. These can affect the accuracy of the calculations, and it is advisable to consult relevant standards, references, or professional engineering expertise to ensure accurate and reliable results.

Calculation Reference
Schaum's Outline - Machine Design

22 Jun 2007
Last Modified
19 Jul 2023
File Size:
File Version:

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Comments: 3
JohnDoyle[Admin] 15 years ago
mstickma that's a useful link for many threadforms. The thread depth variable, t, in the calculation can be set to suit any particular threadform.
JohnDoyle[Admin] 15 years ago
John, Thanks for creating this spreadsheet. It has saved me the time of recreating it from a similar Mathcad file I was using.I was wondering if you could elaborate on what the thread depth variable is in the sheet. I took it as 0.600 X P for metric fine threads. I found this at: http://www.colinusher.info/Livesteam/mewdata.html#dataIs this correct?
JohnDoyle[Admin] 16 years ago
There is a very useful tool on the Norbar website called the Torque Tension Calculator. It provides a useful check and useful thread dimensions which can be inserted into the spreadsheet calculation.