Linear Actuator's Duty Cycle? 1

[Helepolis]

Hello all,

I'm looking at linear actuators and a bit confused about the duty cycle parameter.
Most of them has the duty cycle parameter in the datasheet and it ranges between 10-20% (under load), when few of the manufacturers state this parameter under partial load (Duty Cycle of 50% under 25% load) and some of them exclude this parameter all together, despite using the same general construction and mechanics.

Dose all electrical actuators have duty cycles?
It might be a stupid question but for example, I've disassembled an old veterinarian ventilator that delivers an alternating air flow by pushing a piston using a linear actuator, and that ventilator doesn't have any restrictions on the "ON" time or any specification for the “resting” time.
So basically it seems that the machine doesn't have a duty cycle.

I understand that this parameter is relative, but none of the actuators state the maximum "ON" time, just the 10% or 20% of duty cycle.
So by that logic it doesn't matter if I run an actuator for 10 minutes or 10 hours as long as I let it rest 90% or 80% of the time (depending on the product specifications).
So it doesn’t make any sense, if the duty cycle is there to protect the product from wear and tear, how this parameter can be applied for the same product if the wear is vastly different for 10 minutes or 10 hours of usage.

 

[hendersdc]

I think the duty cycle is more for thermal performance than to prevent wear and tear. For example, if you were to run at 100% duty cycle at full load it would overheat and fail/shutoff.

 

[SwinnyGG]

So by that logic it doesn't matter if I run an actuator for 10 minutes or 10 hours as long as I let it rest 90% or 80% of the time (depending on the product specifications).

I don't think you quite understand what duty cycle means.

Let's say I have a machine with a 100 second cycle time. The linear actuator runs for a total of 10 seconds out of every 100 second cycle.

If I turn that machine on and let it run for one hour, the duty cycle of that machine is 100% for that hour. If I turn the machine on and let it run until the end of time, the duty cycle of the entire machine is still 100%.

In both cases, the duty cycle of the actuator relative to the whole system is 10%. How long the machine is left to run is immaterial.

So it doesn’t make any sense, if the duty cycle is there to protect the product from wear and tear, how this parameter can be applied for the same product if the wear is vastly different for 10 minutes or 10 hours of usage.

The duty cycle limit is not put in place to prevent normal wear of the parts- the duty cycle is there to prevent things like overheating of motors which would cause premature failure. This is a small but subtle difference.

Using your ventilator as an example- I am absolutely sure that a ventilator designed for use in a hospital during surgery or for a patient on life support is designed for 100% duty cycle- meaning it can be turned on and left to run indefinitely.

The actuator that moves the piston is not running 100% of that time- it might be moving down 25% of the time, and then back up 25% of the time, for a total duty cycle of 50%.

The duty cycle of an entire system and the duty cycles of its individual components may be the same, but they are not the same by definition.

 

[EdStainless]

By over designing you can get higher duty ratings. Maybe the ventilator is only loading the actuator to 10% of its max rated load.
There are limits on the 'on' time that a unit can handle.
And all of this is thermally limited.

= = = = = = = = = = = = = = = = = = = =
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[TheTick]

I ran a lab where we tested hundreds of linear actuators from different manufacturers. These were actuators typically used on hospital beds and patient-lifting equipment.

Duty cycle typically prevents failure of mechanical components due to excessive heat. Most linear actuators use a worm drive of some kind. Many have plastic gears. These are the components that typically fail.

 

[Helepolis]

I still not sure how to reffer to this parameter

I don't think you quite understand what duty cycle means.

Let's say I have a machine with a 100 second cycle time. The linear actuator runs for a total of 10 seconds out of every 100 second cycle.

If I turn that machine on and let it run for one hour, the duty cycle of that machine is 100% for that hour. If I turn the machine on and let it run until the end of time, the duty cycle of the entire machine is still 100%.

In both cases, the duty cycle of the actuator is 10%. How long the machine is left to run is immaterial.

This is what confuses me, if the duty cycle of an hour and for eternity is still 10%, why is it relevant? even if it is there to prevent over
heating.
I'll try to rephrase my question, if I have two exact same actuators, with 10% duty cycle, and run them under the same conditions (force, speed, voltage etc.) but one of them runs for 1 minute (with 10 minutes off time) and the other one runs for 100 hours (with 1000 hours off time). Both of them have 10% of duty cycle but the second actuator is stressed way more the first one. What can I infer from this 10% duty cycle?
Because even if maintain the 10% on and 90% off time the chance for the second actuator to fail is way higher than for the first one.
Without a fixed time frame reference this 10% won't give me an indication of how to use the actuator without it failing, as the off time is irrelevant for determining if the actuator will fail.

The actuator that moves the piston is not running 100% of that time- it might be moving down 25% of the time, and then back up 25% of the time, for a total duty cycle of 50%.

How come the total duty cycle of the actuator is 50% if this parameter indicate the ratio between the ON time and OFF time, so while the piston is moving back and forth it can be considered as ON time and OFF time will be when it stopes to change direction.

 

[Helepolis]

By over designing you can get higher duty ratings. Maybe the ventilator is only loading the actuator to 10% of its max rated load.
There are limits on the 'on' time that a unit can handle.
And all of this is thermally limited.

I ran a lab where we tested hundreds of linear actuators from different manufacturers. These were actuators typically used on hospital beds and patient-lifting equipment.

Duty cycle typically prevents failure of mechanical components due to excessive heat. Most linear actuators use a worm drive of some kind. Many have plastic gears. These are the components that typically fail.

By this logic the duty cycle should be presented as a chart that include ON time, Applied Force and a duty cycle for each combination, and not as a single property that describes the ratio between ON and OFF states.

 

[Sparweb]

Are we missing something obvious?
A linear actuator extends or retracts and then comes to the end of the stroke. Then it STOPS.
The time it takes to travel the full stroke is the "quantum" of time you must bear in mind when evaluating duty cycle.

Let's assume it takes the actuator 5 seconds to fully stroke in or out. After a 5 second stroke, you should wait for 20 seconds before moving again to get a 20% duty cycle.

5 / (20+5) = 0.20

STF

 

[IRstuff]

No, it's a combination. The actuation/loading causes heating of the actuator; that heat requires a finite amount of time to dissipate. It takes a certain amount of time to reach maximum temperature, as well.

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[BrianE22]

I agree that you need a maximum on time allowable. If you run it continuously in-out for an hour (then let it rest 5 hours) it will get a lot hotter than if you run it continuously for 10 minutes and let it sit for 50 minutes.

 

[Sparweb]

Sure, but it won't get to max temperature if it's allowed to cool between each stroke.
It does make it sound like the actuator is under-designed, when I put it that way...

STF

 

[Helepolis]

I understand the basic concept of duty cycle (ratio of OFF/ON times), but again, how can I determine the duty cycle if the heating of the actuator is a funtion of ON time and apllied load and the only parameter that is supplied for determining this is that ratio, without any info about the amount of heat produced for each set of ON time and applied load.
As I can produce the same amount of haet when runing the actuator for 10 seconds and for 10 minutes.
There should be provided a graph of heating as a funtion of ON time and load applied, and for each set of those parameters a duty cycle can be deduced.

 

[IRstuff]

There's a thing called "common sense," which would start with the assumption that the maximum two-way traverse time at full load is the maximum time and go from there.

TTFN (ta ta for now)
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[SwinnyGG]

It does make it sound like the actuator is under-designed, when I put it that way...

In my experience, linear actuators are always waaay over designed- because A) the manufacturer usually has a warranty of some kind and B) the buildup of heat (which the duty cycle limit is meant to control) depends heavily on circumstances out of their control, namely the airflow around the actuator body. So they test and determine duty cycle ratings based on worst or near-worst case scenarios, such as the actuator being inside a very small volume that is well insulated with no airflow and no exchange of the hot air, etc.

I'll try to rephrase my question, if I have two exact same actuators, with 10% duty cycle, and run them under the same conditions (force, speed, voltage etc.) but one of them runs for 1 minute (with 10 minutes off time) and the other one runs for 100 hours (with 1000 hours off time). Both of them have 10% of duty cycle but the second actuator is stressed way more the first one. What can I infer from this 10% duty cycle?

If you mean that you run the actuator continuously for 1 minute and then rest for 9 minutes, or 100 hours and then rest for 900 hours, in both cases the duty cycle of the actuator is 100%. Duty cycle at the component level, where you're working here, is based on the component, not the rest of the system.

If the operation of the machine is a square wave function, duty cycle is the width of the peak divided by the period.

IR has it right- these numbers are based on the actuator's real world cycle time.

Any decent manufacturer (a manufacturer, not a reseller who buys offshore actuators and labels them without any real engineering, which is very common) will have a very in-depth table of duty cycle for a given series of actuators.

This table will start at the top with a very short period of time for which the actuator is rated at 100% duty cycle, and then progress through lower percentages and longer periods, until some inflection point at or below which infinite periods of operation are allowed.

 

[TheTick]

Every manufacturer has its own definition of what "real world" testing is. Some are better than others. There is no "industry standard". Most are overly optimistic in their performance projections.

The only way to know for sure is to test on your own.

 

[Nescius]

Of course, the time period matters. It's all about heat dissipation. A duty cycle number doesn't mean anything without that context. A duty cycle could be based on a time period of fractions of a second, or days, depending on the machine.

Constant electrical type loads make it easier to visualize...for example, I believe Miller welders (maybe others, too?) specify the duty cycle as a percentage of a 10 minute period, at a given amperage output.

 

[CWB1]

Duty cycle in recent years has gotten somewhat gimmicky in that some manufacturers selling to consumers will list a high duty cycle without stating a total (usually low) period. Quite often though there's a single duty cycle and period listed in product literature that will apply to most of their target market, for additional duty cycles over different periods you have to call the manufacturer and hope they have the data. For many consumer products - good luck. Manufacturers of industrial products OTOH tend to be pretty good about having and sharing this data.