Servo terminology!!!??? torque vs. holding power???
07-03-2002, 05:30 PM
#1
Thread Starter
Servo terminology!!!??? torque vs. holding power???
Hi guys,
I have had this question in my mind for a while. What is the difference between Torque (oz/in.) and Holding Power (oz/in.)? For example the Hitec 5945 has a rated torque of 180oz/in and holding power is 540oz/in.
thanks,
anurag
I have had this question in my mind for a while. What is the difference between Torque (oz/in.) and Holding Power (oz/in.)? For example the Hitec 5945 has a rated torque of 180oz/in and holding power is 540oz/in.
thanks,
anurag
07-04-2002, 05:04 AM
#2
Senior Member
My Feedback: (12)
In Layman's terms
OK, I'll give this a shot.
My understanding is as follows:
Maximum torque = The amount of force which can be applied against the servo arm and the servo arm will continue to move in the requested direction. Any additional force will cause the servo to stall and the arm will stop moving.
Holding power = Once the servo arm has arrived at its requested position, the amount of force that can be applied to the servo arm without forcing the arm to move from the requested position. Any additional force will cause the servo arm out of position.
Why can Holding power be much greater than Torque?
For torque, the motor shaft drives the output arm through reduction gears, much like a spring in a clock drives the hour hand through a set of reduction gears. Let's assume the hour hand is locked on the shaft and cannot slip. Now if you apply enough force against the hour hand you can cause it to stop moving. This would be the maximum torque of the clock movement. Now, how much additional force would you need to apply to the hour hand to cause it to move backward and force the gears and spring into reverse? This would be the holding power of the clock movement.
For torque, the motor is driving the output shaft through gears which give it a great mechanical advantage.
For Holding power, the output shaft is driving the motor through gears which give it a great mechanical dis-advantage.
Does this help ??
My understanding is as follows:
Maximum torque = The amount of force which can be applied against the servo arm and the servo arm will continue to move in the requested direction. Any additional force will cause the servo to stall and the arm will stop moving.
Holding power = Once the servo arm has arrived at its requested position, the amount of force that can be applied to the servo arm without forcing the arm to move from the requested position. Any additional force will cause the servo arm out of position.
Why can Holding power be much greater than Torque?
For torque, the motor shaft drives the output arm through reduction gears, much like a spring in a clock drives the hour hand through a set of reduction gears. Let's assume the hour hand is locked on the shaft and cannot slip. Now if you apply enough force against the hour hand you can cause it to stop moving. This would be the maximum torque of the clock movement. Now, how much additional force would you need to apply to the hour hand to cause it to move backward and force the gears and spring into reverse? This would be the holding power of the clock movement.
For torque, the motor is driving the output shaft through gears which give it a great mechanical advantage.
For Holding power, the output shaft is driving the motor through gears which give it a great mechanical dis-advantage.
Does this help ??
07-04-2002, 08:04 AM
#3
Thread Starter
Servo terminology!!!??? torque vs. holding power???
Yes, this does make sense...One more question though...in case of the holding power...if the servo has metal gears vis a vis non-metal, will the holding power be more???? since it is harder to strip a metal gear than a non-metal gear??? One second thought, you did not say strip the gears into reverse..u said move the servo motor into reverse...so I guess my question is answered already 
thanks for the explanation though.
Anurag
thanks for the explanation though.
Anurag
07-04-2002, 01:23 PM
#4
My Feedback: (31)
Servo terminology!!!??? torque vs. holding power???
Your definition sounds good to me less the statement about regarding holding power and and mechanical dis-advantage.
How can the servo be at a mechanical dis-advantage while holding if it's in an advantage while moving??? A disadvantage would mean the torque has is reduced or of a lower capability.
Typically the term mechanical advantage is a term used to associate an advantage over or of one lever arm arm setup as compared to another. A ratio 1:1 verses 1.5:1 for example. For our purposes a servo arm that is shorter than the combined length of the hinge center line to the control horn pivot point of the surface in question would give a mechanical advantage.
Lets say 1" servo arm and control arm/horn length of 1-1/2" would have a mechanical advantage. Inversely 1-1/2" servo arm and a 1" control arm/horn would be at a mechanical disadvantage and should be avoided at all cost's.
The holding torque is higher relative to the electric motor being stalled where the greatest torque is available from an electric motor.
How can the servo be at a mechanical dis-advantage while holding if it's in an advantage while moving??? A disadvantage would mean the torque has is reduced or of a lower capability.
Typically the term mechanical advantage is a term used to associate an advantage over or of one lever arm arm setup as compared to another. A ratio 1:1 verses 1.5:1 for example. For our purposes a servo arm that is shorter than the combined length of the hinge center line to the control horn pivot point of the surface in question would give a mechanical advantage.
Lets say 1" servo arm and control arm/horn length of 1-1/2" would have a mechanical advantage. Inversely 1-1/2" servo arm and a 1" control arm/horn would be at a mechanical disadvantage and should be avoided at all cost's.
The holding torque is higher relative to the electric motor being stalled where the greatest torque is available from an electric motor.
07-04-2002, 04:02 PM
#5
Senior Member
My Feedback: (12)
Advantage vrs Dis-advantage
These terms have to be applied relative to the driving force.
In describing Torque, the motor is driving the output shaft. Lets say the ratio (don't have any idea what the actual ratio is
) is 50 to 1, meaning for every 50 revolutions of the motor, the output shaft turns 1 revolution. Clearly a mechanical advantage for the motor.
Now, for Holding power the motor is stationary ( being held electronically) and the output shaft is trying to drive the motor in reverse. The ratio is now 1 to 50 for shaft to motor, clearly a mechanical dis-advantage for the output shaft.
Does that make sense ???
In describing Torque, the motor is driving the output shaft. Lets say the ratio (don't have any idea what the actual ratio is
) is 50 to 1, meaning for every 50 revolutions of the motor, the output shaft turns 1 revolution. Clearly a mechanical advantage for the motor.Now, for Holding power the motor is stationary ( being held electronically) and the output shaft is trying to drive the motor in reverse. The ratio is now 1 to 50 for shaft to motor, clearly a mechanical dis-advantage for the output shaft.
Does that make sense ???
07-04-2002, 06:24 PM
#6
My Feedback: (31)
Servo terminology!!!??? torque vs. holding power???
For Holding power, the output shaft is driving the motor through gears which give it a great mechanical dis-advantage.
Holding power is irrelevant in this example. Holding power is derived from the stalled motor. It's resistant force. If the aero-load overcomes the holding torque and the mechanical disadvantage comes to play via the gear-train it's at a disadvantage to overcome the positive torque output of the servo's geatrtrain... 1:50 is a tough, 50:1 one is easy...
Sounds like we have is a contradiction of terms or understanding.
First off as mentioned previously, holding torque/power is a function of the motor being stalled. Nothing more. Weather or not the gear-train is driven by the motor or pushed by aero-loads.
The torque is always a constant. T=FxD. torque = force x distance. Force is measured at the end of the arm and is multiplied by the distance from the pivot points to obtain a torque rating. This torque rating is able to overcome, move or twist a static load up to it's rating. This is a function in action not stalled.
In all actual practice if the mechanical disadvantage existed it would be a good thing.... A disadvantage would not allow the the aero-loads to easily over-power the servo's holding ability. If the servo motor failed and allowed the aero-load to move the output arm, an advantage positive or negative would be moot.
Holding power is irrelevant in this example. Holding power is derived from the stalled motor. It's resistant force. If the aero-load overcomes the holding torque and the mechanical disadvantage comes to play via the gear-train it's at a disadvantage to overcome the positive torque output of the servo's geatrtrain... 1:50 is a tough, 50:1 one is easy...
Sounds like we have is a contradiction of terms or understanding.
First off as mentioned previously, holding torque/power is a function of the motor being stalled. Nothing more. Weather or not the gear-train is driven by the motor or pushed by aero-loads.
The torque is always a constant. T=FxD. torque = force x distance. Force is measured at the end of the arm and is multiplied by the distance from the pivot points to obtain a torque rating. This torque rating is able to overcome, move or twist a static load up to it's rating. This is a function in action not stalled.
In all actual practice if the mechanical disadvantage existed it would be a good thing.... A disadvantage would not allow the the aero-loads to easily over-power the servo's holding ability. If the servo motor failed and allowed the aero-load to move the output arm, an advantage positive or negative would be moot.
07-04-2002, 07:01 PM
#7
Senior Member
My Feedback: (12)
Same thing, only different
I think we are saying the same thing, only with different perspectives.
Look at it this way:
Assume there is no mechanical advantage involved, ratio is 1 to 1.
Now assume you apply enough force to the servo arm to stall it. The servo motor is putting out its maximum torque, but is no longer turning. Now if ANY additional force is applied to the servo arm, the motor shaft will immediately be forced back-wards.
But with a 1 to 50 ratio you would have to apply considerable more force to drive the motor shaft back-wards.
This is simple laws of Physics.
Look at it this way:
Assume there is no mechanical advantage involved, ratio is 1 to 1.
Now assume you apply enough force to the servo arm to stall it. The servo motor is putting out its maximum torque, but is no longer turning. Now if ANY additional force is applied to the servo arm, the motor shaft will immediately be forced back-wards.
But with a 1 to 50 ratio you would have to apply considerable more force to drive the motor shaft back-wards.
This is simple laws of Physics.
07-05-2002, 05:39 AM
#8
My Feedback: (31)
Servo terminology!!!??? torque vs. holding power???
Are we having fun yet????
I agree it's physic's. There are other factors at work and I just don't think in pans out with this example. Yes it exist's but does it contribute to the holding power of the stalled servo motor???
The servo or motor/rotor/core is already stalled unless the surface is moving... We agree the highest available torque is available while stalled.
I am sure knowing full well you should'nt you moved a servo/surface mechanically by hand at one time or another in your dealings with models... I know I have, more than once too... My recollection is it's very easy to do... It's much harder if not nearly impossible without damaging something to attempt the same manual movement of the surface when the servo is powered up...
I agree it's physic's. There are other factors at work and I just don't think in pans out with this example. Yes it exist's but does it contribute to the holding power of the stalled servo motor???
The servo or motor/rotor/core is already stalled unless the surface is moving... We agree the highest available torque is available while stalled.
I am sure knowing full well you should'nt you moved a servo/surface mechanically by hand at one time or another in your dealings with models... I know I have, more than once too... My recollection is it's very easy to do... It's much harder if not nearly impossible without damaging something to attempt the same manual movement of the surface when the servo is powered up...
07-05-2002, 01:34 PM
#9
Senior Member
My Feedback: (12)
I agree
Yes, I agree, there is much more technology in play here. But as stated in my first post, I was trying to keep it simple, hence the title ---
"In Layman's terms".
In fact your last post serves to prove my point. Move the arm on a non-powered servo. Sure you can easily do it, but there is some amount of resistance there. Now, take a dis-assembled servo and spin the motor shaft -- much easier ins't it ?
Technical discussions are always FUN.
"In Layman's terms".
In fact your last post serves to prove my point. Move the arm on a non-powered servo. Sure you can easily do it, but there is some amount of resistance there. Now, take a dis-assembled servo and spin the motor shaft -- much easier ins't it ?
Technical discussions are always FUN.
