BIZOMAT MAKEOS · Mechanics
Scissor Lift Screw Drive Check (Actuator Force Through the Lift, Screw Torque, Lift Time, Power)
For a single-stage scissor lift table raised by a horizontal lead screw, what force does the screw push with at the bottom of the lift, what torque and power does it need, and how long does the lift take?
Check of a scissor lift table driven by a horizontal screw between the bottom pivots: the platform height range from the arm length and angles, the horizontal actuator force from the load (largest at the lowest angle), the screw travel, the torque to drive the screw at the largest force (as the power screw torque tool), the lift time at the screw speed and the drive power, against the torque available and the time allowed.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Load including the platform | N | Load on the platform plus the platform itself (arm weight left out). |
| Scissor arm length (pivot to pivot) | mm | Length of each scissor arm between its end pivots. |
| Lowest arm angle from horizontal | ° | Arm angle from horizontal with the table lowered. |
| Highest arm angle from horizontal | ° | Arm angle with the table raised. |
| Screw | ||
| Mean (pitch) diameter of thread | mm | Mean diameter of the screw thread. |
| Lead | mm | Lead of the screw. |
| Thread form | choice | Thread form. |
| Thread friction coefficient | Thread friction (a ball screw has far less). | |
| Mean collar diameter (0 = rolling thrust bearing) | mm | Mean diameter of a plain thrust collar (0 for a rolling thrust bearing). |
| Collar friction coefficient | Collar friction. | |
| Drive | ||
| Screw speed | rpm | Speed of the screw. |
| Torque available at the screw | N·m | Torque the motor and gearing can give at the screw. |
| Longest lift time | s | Longest time allowed for a full lift. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Load including the platform
- What it is
- The vertical load on the platform, including the platform itself, that the screw drive must raise.
- Units and format
- Enter a number in N: newtons. The form accepts 0.000001 N to 100000000 N. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 5000 N as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Screw force and torque rise in direct proportion.
- Get it right
- Enter newtons, not kilograms; the tool leaves out the weight of the scissor arms themselves.
What this means: Scissor arm length (pivot to pivot)
- What it is
- The length of each scissor arm between its two end pivots.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 100000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1200 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Longer arms give more height and need more screw travel for the same angles.
- Get it right
- Measure pin centre to pin centre, not the overall arm length.
What this means: Lowest arm angle from horizontal
- What it is
- The angle of the scissor arms above horizontal with the table fully lowered. The screw force is highest here.
- Units and format
- Enter a number in °: degrees of angle. The form accepts 1° to 80°. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 10° as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The largest screw force varies with 1 ÷ tan of this angle, rising steeply as it gets small.
- Get it right
- Measure from horizontal; small angles give very large forces, so measure carefully.
What this means: Highest arm angle from horizontal
- What it is
- The angle of the scissor arms above horizontal with the table fully raised.
- Units and format
- Enter a number in °: degrees of angle. The form accepts 2° to 85°. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 50° as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- It must exceed the lowest angle; measure from horizontal.
Screw
What this means: Mean (pitch) diameter of thread
- What it is
- The mean or pitch diameter of the lifting screw thread, where the thread force is taken to act.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 10000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 25 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Use the pitch diameter, not the nominal outside diameter.
What this means: Lead
- What it is
- How far the screw nut moves along the screw in one revolution. It equals the pitch times the number of starts.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 1000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 5 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A larger lead shortens the lift time but raises the torque.
- Get it right
- For a multi-start screw, multiply the pitch by the number of starts.
What this means: Thread form
- What it is
- The shape of the lifting screw thread. Inclined flanks add friction compared with a square thread.
- Units and format
- Choose one of 3 options from the list: Square; Acme (29°); Trapezoidal (30°).
- Example
- The form opens with “Trapezoidal (30°)” selected as a worked example. Choose the option that matches your case.
- Get it right
- Check the thread standard on the screw drawing or catalogue.
What this means: Thread friction coefficient
- What it is
- The friction coefficient between the screw thread and its nut.
- Units and format
- Enter a plain number with no unit. The form accepts 0 to 1. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.1 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Higher friction raises the screw torque.
- Get it right
- A ball screw has far less friction than a sliding nut; use the supplier’s value for the actual nut.
What this means: Mean collar diameter (0 = rolling thrust bearin…
- What it is
- The mean diameter of a plain thrust collar that carries the screw’s axial force. Enter 0 for a rolling thrust bearing, which adds no collar friction here.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0 mm to 10000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- 0 means no collar friction is counted; use it only if a rolling bearing takes the thrust.
What this means: Collar friction coefficient
- What it is
- The friction coefficient of the thrust collar under the screw force.
- Units and format
- Enter a plain number with no unit. The form accepts 0 to 1. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.01 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- It has no effect when the collar diameter is 0.
Drive
What this means: Screw speed
- What it is
- The rotational speed of the lifting screw during the lift, which with the lead sets how fast the nut travels.
- Units and format
- Enter a number in rpm: revolutions per minute. The form accepts 0.000001 rpm to 100000 rpm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 300 rpm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A faster screw shortens the lift time and raises the power needed.
- Get it right
- Use screw speed after any gearbox, not motor speed.
What this means: Torque available at the screw
- What it is
- The torque the motor and gearing can deliver at the screw, compared with the largest torque the lift needs.
- Units and format
- Enter a number in N·m: newton-metres. The form accepts 0.000001 N·m to 10000000 N·m. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 80 N·m as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Use the torque at the screw speed being used, after gearbox losses.
What this means: Longest lift time
- What it is
- The longest time you accept for a full lift from lowest to highest position.
- Units and format
- Enter a number in s: seconds. The form accepts 0.000001 s to 100000 s. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 20 s as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Enter seconds, not minutes.
Method and formulas
- Height = L sin θ; base length = L cos θ; screw travel = L (cos θ_min − cos θ_max).
- Virtual work: W dh = F dx gives F = W ÷ tan θ, largest at the lowest angle.
- Torque to raise the largest force (as the power screw torque tool); lift time = travel ÷ (lead × speed); power = torque × ω.
- Example: 28,356 N at the bottom of the lift needs 59.654 N·m; the lift takes 16.417 s.
Limitations
- Single stage with the load centred and arm weight and pivot friction left out.
- Screw buckling and nut wear not checked.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Size the screw and motor for a scissor table
- See why scissor lifts start hard at the bottom
- Check a lift table for a heavier load
Who it is for
- Machine designers
- Design engineers
- Materials handling engineers
What you get
- Height range and screw travel
- Largest actuator force and screw torque
- Lift time and power
Interpreting the result
- The force at the bottom of the lift is several times the load; starting a little higher (a larger lowest angle) cuts it sharply.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 153–163, 1921–1928. Topic reference; no source tables or recommended settings are reproduced.
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