Enter Lift Parameters
Use measured equipment geometry whenever possible.
Example Data Table
These sample cases show how angle and reach affect calculated values. They are examples only.
| Case | Combined Mass | Sling Angle | Dynamic Load | Strap Tension | Boom Moment |
|---|---|---|---|---|---|
| Balanced transfer | 85 kg | 52° | 958 N | 304 N | 609 N·m |
| Lower sling angle | 85 kg | 35° | 958 N | 418 N | 609 N·m |
| Longer horizontal reach | 85 kg | 52° | 958 N | 304 N | 759 N·m |
Formula Used
The calculator applies static equilibrium with an adjustable dynamic multiplier. It assumes a centred load and equal sharing across active sling straps.
W = (mpatient + mequipment) × gWdesign = W × DTstrap = Wdesign ÷ [n × sin(θ)]Hstrap = Tstrap × cos(θ)Mboom = Wdesign × [L × cos(β)]Factuator, ideal = Mboom ÷ r
Where: m is mass, g is gravity, D is dynamic factor, n is strap count, θ is sling angle, L is boom length, β is boom angle, and r is actuator moment arm.
How to Use This Calculator
- Enter the patient mass and all carried sling hardware mass.
- Choose the number of straps that are actually supporting the load.
- Measure the sling angle from the horizontal line.
- Enter the boom length and current boom elevation angle.
- Use a documented perpendicular actuator moment arm when available.
- Select a conservative dynamic factor for movement and start-up loading.
- Calculate, then compare every result with manufacturer limits and procedures.
Understanding Hoyer Lift Forces
Why transfer force matters
A mobile lift supports more than patient weight. The sling, spreader bar, clips, and attachments add mass. Their combined mass becomes vertical load when multiplied by gravity. The lift frame must carry that load through several parts. Those parts include straps, hooks, the boom, the mast, and the actuator. A force estimate helps explain why geometry matters. It also shows why safe working limits must never be treated as rough suggestions.
Start with the total supported mass
The first calculation adds patient mass and equipment mass. This creates the supported mass. Multiply that mass by gravitational acceleration. The result is the static vertical load in newtons. Static load describes an ideal steady lift. Real transfers are rarely perfectly steady. A lift can start abruptly. A caregiver can reposition a patient. The sling can settle under tension. The dynamic factor allows extra allowance for these conditions. It is not a replacement for a manufacturer rating.
Sling angles change strap tension
Each active sling strap supports part of the design load. The share depends strongly on sling angle. A higher angle places the straps closer to vertical. This reduces tension in each strap. A lower angle creates a flatter strap path. Flatter straps need more tension to provide the same vertical support. The horizontal pull also rises as the sling becomes flatter. In a symmetrical arrangement, opposing horizontal pulls may cancel at the centre. Yet each individual strap, hook, and connection still carries its own force.
Boom reach creates a turning effect
The boom carries the load away from the mast pivot. This produces a bending moment. A longer horizontal reach increases that moment. Raising the boom reduces the horizontal reach. That can reduce the calculated bending moment. The geometry of a real lift is more complex than a simple beam. Joints, brackets, cylinders, and frame stiffness all matter. Still, the moment calculation is useful for comparing positions. It helps show why load location changes mechanical demand. Check caster position before interpreting boom results. Floor slope, mast tilt, and wheel orientation can change stability. These conditions may not directly change the simple force equations. They can still affect safe operation.
Actuator demand needs careful interpretation
The actuator value is an idealized force. It divides boom moment by the selected perpendicular moment arm. Actual hydraulic or electric actuator force depends on linkage angles. Friction, cylinder attachment points, and internal efficiency also change it. Use manufacturer drawings for equipment design work. Never select a replacement actuator from this estimate alone. A qualified engineer should review any modification, repair, overload event, or unusual transfer setup.
Use calculated results with operational checks
Inspect the lift before use. Confirm the sling is compatible with the spreader bar. Verify every clip is fully engaged. Keep the load centred and controlled. Follow the stated patient weight capacity. Use trained caregivers and the approved transfer plan. Stop immediately if equipment behaves abnormally. Calculations support safer decisions, but they cannot verify patient comfort, medical suitability, component wear, or hidden damage. Safe transfers depend on both numbers and careful practice.
Frequently Asked Questions
1. What does this calculator estimate?
It estimates vertical load, sling strap tension, horizontal strap pull, boom moment, and ideal actuator force. It uses entered mass, angles, geometry, and a dynamic load factor.
2. Why are results shown in newtons?
Newtons are standard force units. Mass becomes force when multiplied by gravitational acceleration. The calculator also shows equivalent kilogram-force for easier comparison with familiar weight values.
3. What sling angle should I enter?
Measure the angle between one loaded sling strap and a horizontal reference line. Use the actual transfer position. Avoid estimating from an unloaded sling because its angle changes under load.
4. Why does a lower angle increase strap tension?
A flatter strap provides less vertical support per unit of tension. Each strap therefore needs greater tension to support the same vertical load. Horizontal pull also increases.
5. Does equal load sharing always occur?
No. This is a simplifying assumption. Uneven seating, different strap lengths, patient posture, and off-centre lifting can shift load between straps. Use conservative procedures and approved sling configurations.
6. What dynamic factor should I use?
Use 1.00 only for an ideal static comparison. Use a conservative value based on your engineering procedure, movement conditions, and manufacturer guidance. Never use this factor to override rated limits.
7. Is ideal actuator force the actual cylinder force?
Not necessarily. It is a simplified equilibrium estimate. Actual force depends on linkage geometry, cylinder angle, friction, efficiency, and mechanical stops. Use documented equipment geometry for formal analysis.
8. Can I use this page to select a lift?
No. Choose lift equipment using the manufacturer’s safe working load, approved sling compatibility, clinical assessment, and institutional procedures. This calculation is supplementary planning information.
9. What is boom bending moment?
It is the turning effect produced by the vertical design load at the boom’s horizontal reach from the mast pivot. Greater reach increases moment and structural demand.
10. Should equipment mass be included?
Yes. Include the sling, spreader bar, clips, scales, and attachments carried by the hoist. Excluding them understates the supported load and calculated forces.
11. When should a qualified professional review the setup?
Seek qualified review for equipment modification, repair, unusual patient positioning, overload concerns, recurring faults, or any mismatch between calculated forces and published equipment limits.