rotational motion into linear motion – force calculation?

[ell81Participant @ell81]

So how can you convert the hp of a rotating shaft into linear force when making something similar to a linear actuator?What is the calculation please?

Thanks.

 

[Robert Atkinson 2Participant @robertatkinson2]

Well HP is defined as a linear movement. 1HP is lifting 550lbs 1 ft in 1 second.
If using a screw to cinvert you have to account for losses.

[Julie AnnParticipant @julieann]

The question doesn’t make any sense. Horsepower is power, which is the rate of doing work, ie, time is involved. Whereas a force is just that, a force which is static and not time dependent. The OP should reformulate the question in a form that makes physical sense. Then we can answer it.

Julie

[John HaineParticipant @johnhaine32865]

If you had a motor turning a drum with a circumference of 1 foot at 550 rev per second (3300 rpm) with a string round the drum lifting a one pound weight, the motor would be outputting 550 foot-pounds per second which is one horsepower.  Hopefully that will give you a feel for magnitude and how to do the calculations.  Best not to use ancient units though!

[Howard LewisParticipant @howardlewis46836]

You could rotate a screw thread to produce a linear force. (Think car jack!)

But the force will be determined by the pitch of the thread, and coefficient of friction between internal and external parts of the thread

The question might be easier to answer if the OP provided more detail, and the reason for the request.

Howard

[gerry maddenParticipant @gerrymadden53711]

power= torque x ang.velocity = force x linear velocity

(assuming 100% efficiency of conversion.)

Gerry

 

[gerry maddenParticipant @gerrymadden53711]

If I hadn’t replied so late at night I would have added the units which for the above, which would be:-

Watts = (Nm) x (6.28xRPM/60) = (N) x (m/s)

The angular velocity has to be in radians per second, hence the conversion above if you only have RPM to hand.

Gerry

[SillyOldDufferModerator @sillyoldduffer]

Before reading on, it may help to know that ‘Energy’, ‘Force’, ‘Work’ and ‘Power’ have technical definitions that aren’t the same as ordinary English.  And ‘mass’ and ‘weight’ aren’t quite the same thing.   Look them up if confused!

The relationship is this:

• a certain amount of energy is needed to provide a force.  Many forces –  reaction, thrust, magnetic, spring, tension, upthrust etc,  but they’re all defined the same way.  Force is whatever is required to accelerate a 1kg mass by 1meter per second per second.  It’s measured in Newtons.
• Work is defined as the energy needed to move a mass a certain distance.  Measured in Joules, where a Joule is 1Newton.metre.    Note the absence of time in the definition of work.  There is no concept of acceleration or how quickly the work must be done.
• Power introduces time. It’s the rate of doing work.  Power is measured in Watts, and 1 watt is one Joule per Second. Calculating the power needed in Watts allows the motor to be selected.
• If needed, the calorific value of fuels is known in Joules, so the designer can calculate how big the fuel tank has to be. Or battery. He can also balance performance vs efficiency, and work out the best that can be done within a budget.

These steps and relationships provide the answer.

Looking at a car engine and gearbox might help and it illustrates some practical problems.

• The car engine has a certain power output, say 50000 watts, ideally at 2500rpm.  Lower than 800rpm will probably stall it, whilst red-lining soon destroys the engine.  Electric motors also have restrictions, and sometimes it’s important to choose carefully.  Hold that thought for later!
• A car has mass (loosely weight).  Ignoring friction, moving a 1000kg car 1000 metres takes 1000000 Joules.   Mass and distance provide a toe-hold on work.
• From Joules we can calculate the size of engine needed to accelerate the car in Watts.  Acceleration is rate of work: whilst a modest 50kW engine in a family saloon might take 15 seconds do 0-70mph, a 400kW engine might achieve under 4 seconds.  Cruising is mostly about overcoming friction, and requires less power than starting, accelerating or hill-climbing.
• Movement is achieved by the engine spinning the road wheels, but because motors are imperfect a gearbox is needed to match the motor to the amount of work involved and how quickly that work has to be done.  Work defined  in Joules, not English:
  • To get the car moving from a standing start, especially on a hill, a low gear is needed.   The engine spins fast, transferring enough force to turn the road-wheels slowly, overcoming stiction and inertia.  Rule of thumb, the Watts needed to do this may be enough for everything else, unless we are a racer.
  • Once the car is moving, the car can gear up to go faster.   Top gear (4th), normally 1:1, covers from about 25 to 40mph.  Below 25mph, the engine needs a lower gear. For cruising above 40mph, it pays to have an overdrive gear (5th).  Lower gears are needed to climb hills because the engine has to lift the car’s mass vertically, more work.  Steep hills are climbed slowly in low gear with the engine at high rpm.   To accelerate faster whilst overtaking a driver might go down a gear to spin the engine faster.   A heavy lorry has more gears than a car because the mass is much larger, and more control is needed to avoid stalling and to minimise fuel consumption.
• Real designs have to allow for friction, which wastes a lot of energy.  After applying the formula, it’s necessary to add a fudge factor based on practical experience.
• The same maths works for trains, planes, ships, garage doors, rockets, lifts, and anything else where a mass is moved by a force.  Hint, the calculations are easier in metric than Imperial.  Though the physics is identical, Imperial complicates the sums!

So, how heavy is the object being moved by the actuator, how fast must it accelerate, how fast must it move once started, and over what distance?   Is the mass lifted or moved along the flat?

Fingers crossed, I think I’ve identified all the inputs.  If not someone will point out the mistakes!

For completeness, possible to solve the problem experimentally, ideally based on relevant previous experience. Someone who has done this before is gold-dust.  And it’s a quick way of confirming simple designs, or helping a learner muddle through!   Otherwise, experiment works, but can’t achieve the same efficiencies as a calculated design, doesn’t scale-up, or deal with innovations.  Very costly when a complex design has to evolve from scratch, because that can break the bank!  After a certain level, it’s much cheaper to debug on paper or in a computer than to build a long series of prototypes.

Dave

[duncan webster 1Participant @duncanwebster1]

Oh dear, where do I start?

• a certain amount of energy is needed to provide a force: No it isn’t, force doesn’t involve energy unless what the force is acting on is moving. My cup of tea is sat on the table and gravity is causing a force to act on this mass, no energy involved
• Work is defined as the energy needed to move a mass a certain distance: No it isn’t, energy is force * distance. The force might be the result of gravity acting on a mass, but could just be me pushing  against a spring.
• moving a 1000kg car 1000 metres takes 1000000 Joules. Nope, in the absence of friction or rolling resistance, moving a 1000kg car horizontally would require energy to get it moving, how much depending on how quickly you want it to move, but you could recoup that energy when it slowed down, so net zero. Lifting it vertically you’d need to include the effect of gravity, which exerts 9.81 N/kg, so 9.81 * 1000  * 1000 = 9810000 Joules
• …..Acceleration is rate of work: Nope acceleration is rate of change of velocity

I’ll call it a day at that

 

[Martin KyteParticipant @martinkyte99762]

Regarding the cup of tea on the table I would agree that the system is in balance so no work is being done but the gravity field has energy and is acting on the mass to create a force.

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