Designing a Patient Hoist Winch: Factors of Safety and Power Calculations

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Discussion Overview

The discussion revolves around the design and calculations for a patient hoist winch, focusing on factors of safety, power requirements, gearbox ratios, and material considerations. Participants are exploring the necessary parameters for safe and effective operation in a university project context.

Discussion Character

  • Homework-related
  • Technical explanation
  • Exploratory
  • Debate/contested

Main Points Raised

  • One participant seeks guidance on determining an appropriate factor of safety for a patient hoist winch designed to lift 180 kg, with an output power of 0.75 kW.
  • Another participant emphasizes the importance of considering human and financial consequences of failure when selecting a safety factor, suggesting that social and economic implications should be factored in.
  • A participant expresses the need for a normal factor of safety to apply to their calculations, indicating that their output force of 1.8 kN does not yet include this factor.
  • One reply challenges the original poster to think critically about the selection of a safety factor rather than simply applying a formula, prompting a deeper consideration of the reasons behind the factor.
  • A participant shares their calculations for determining gearbox ratios, drum size, and rope size, using a factor of safety of 1.5 to calculate the required rope strength.
  • Concerns are raised about the high gear ratio required for the motor and the implications of drum size on this ratio, with a participant questioning how to effectively manage the design given these constraints.

Areas of Agreement / Disagreement

Participants express differing views on the appropriate factor of safety and the implications of design choices, particularly regarding the balance between safety, efficiency, and practicality. The discussion remains unresolved with multiple competing perspectives on the calculations and design parameters.

Contextual Notes

Participants mention various assumptions in their calculations, such as efficiency rates and material properties, but these assumptions are not universally agreed upon. There are also unresolved questions regarding the calculations of input power and RPM, as well as the implications of drum size on gear ratios.

Who May Find This Useful

This discussion may be useful for engineering students, particularly those involved in mechanical design projects, as well as professionals interested in safety factors and power calculations in machinery design.

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Homework Statement


I am trying to design and manufacture a patient moving hoist winch to move patients from wheelchairs to beds etc. The design have to has a safe working load of 180 kg and lifting speed 25mm/sec. I have to use factor of safety so here is my question. How can i find a recommended factor of safety for this type of machine?


Homework Equations


I have the output Force which is 1.8KN, Output speed which is 25mm/sec therefore my output power which is 0.75kw.
The main idea of the problem is to design the winch for this machine. So i have to find all the data first to be able to find the input-power,torque,speed, drum size, rope size etc. I will design also a gearbox in this system, which my ratios will found from the input torque and input speed.


The Attempt at a Solution


I found the output power which is 0.75Kw but this is without factor of safety. Also i know that my input power will be 95% more efficient that output power because of friction loses etc. Any help will be appreciated
 
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What are the human and financial consequences of failure? I don't know where you find the official figure (perhaps in a british standard or similar) but I am asking you to name your own figure. Although the safety factor has to take account of material variations and cover assumptions made in the modelling, the social and economic consequences are arguably greater, especially if the product went into mass production.
 
It is for human. I will just design it for a module in university. What is a normal factor of safety to start with. I have to use factor of safety in my output force to start with. 1.8KN is without factor of safety and i have to change it to a force which will include the factor of safety general for this machine
 
If you are at university, the last thing you should be doing is to "plug a number into a formula". That is why I have asked you to think. What do you think would be a suitable factor of safety? And have you considered all the reasons for having such a factor?
 
I am trying to find the gearbox ratio of this. My calculations were changed so i will write them as i did them and i will ask my question. I am doing a winch for patient moving hoist and i want to determine my drum size, rope size, gear ratio and then to find a motor and with a gearbox who i will design i will achieve the gear ratio who i found.

Output Force: 1.8KN, Output Speed: 25mm/s-> Therefore output power is 45watts. Then my input force it will be ~95% more efficient so 50watts approx.

To find the rope size using 1.5 factor of safety: 180kg*9.81*1.5=2648Kn and from a table which has rope strengths i choose the 3mm rope

Drum size:

I can use the minimum 12*d as my drum diameter. d is the rope diameter
therefore 12*3=36mm drum diameter.

To find the turns using 2m rope length i did N=(2)/(π*0.036)= 17.68 turns to fold it. π=3.14

therefore 17.68*3=0.053m which is the length of my drum

and i want to find my input speed(rpm) to find my gear ratio. i try((0.025)/(π*0.036))*60=13

Here is the problem. According to motors the minimum rpm who i found to use a 50watts motor produces ~3000 rpm which means my gear ratio will be ~230:1. This gear ratio it will be ok if use bigger drum size. My drum diameter is just 3.6cm and the minimum motor shaft i found is 2.2cm. Where to put that all gearbox reductions for that huge ratio? So i think to change my drum size but as my drum size diameter increasing my gear ratio also increasing. What to do? Thank you very much. If anyone can help me to get out of this stuck i will appreciate it. I think that i have wrong on calculating my input power and my rpm. Any help?
 
Last edited:
Any help?
 

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