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Onan Mechanical Governors, Torque and Springs


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  #1  
Old 02-28-2012, 12:44:15 PM
Magnetite Magnetite is offline
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Default Onan Mechanical Governors, Torque and Springs

OK, I know this is sort of a weird way to ask this, but sometimes I have to sort of work my way through a problem like this

I'm trying to understand how the spring governor works on my 6.5NHD. Is there a write-up anywhere about spring governors, other than the manual?

Here's my issue:
The governor arm is connected at the tip to the throttle. At angle1, the throttle is closed (no load). At angle2, the throttle is fully open (full load). The governor arm is rotated towards the closed position by a torque from the governor that increases as the RPM increases.

If the gen is perfectly adjusted and has the correct spring installed all as per the manual, it will be at RPM1=62Hz at no load and RPM2=58Hz at full load. The torque exerted on the governor arm at no load, RPM1, angle1, will be torque1 and at full load, RPM 2, angle2, it will be torque2. torque 1 is greater than torque 2 and is opposite to the pull of the spring.

The spring pulls against the governor arm at a variable location that is a distance R from the governor shaft. If we assume small angles, then the distance the arm moves (and the spring stretches) at point R is proportional to R times the angle (R x angle) the arm moves. The spring has a spring constant k and exerts a force of k times the distance it's stretched, so the force is k x R x angle.

The torque on the governor arm due to that force is applied at distance R, so the torque is proportional to k x Rsquared x angle

I've ignored the zero points and scaling constants, but we adjust the back end of the spring as the manual says so that at no load, the spring force produces a torque on the governor arm that exactly balances the governor torque at no load 62 Hz. The governor torque is at its maximum torque1 here. The spring is countering that by being stretched the maximum. (BTW, note that this is why the governor can never be adjusted to always hold exactly 60 Hz - it has to have a different arm position and different torque between zero throttle and full throttle).

Now, what happens if we add a load? The engine slows, the governor produces less torque at th lower RPM, and the spring pulls the the governor arm towards it, which opens the throttle. Ideally, the throttle will be fully open at full load. In my case it is not fully open. What can I do to fix this? I need to change the rate at which the spring force drops off as the angle of the governor arm changes. I need to change something so that the spring produces the same torque at no load (angle1), but produces more torque than it currently does at angle2).

How does the manual say to fix this? The manual says to turn the sensitivity control CCW (and rezero the no load RPM to 62 Hz). The effect of that is to decrease R (the spring attachment point to the governor arm moves towards the governor shaft).

From above, the torque produced by the spring at any angle is proportional to k x Rsquared x angle

So as you decrease R, you decrease the product of spring constant times R squared, and that is roughly equivalent to decreasing the spring constant and not moving the location of the spring attachment point. This all makes sense to me. The pull of the spring is dropping off too fast on my gen. If I set it correctly to produce 62Hz at no load, and if at full load I'm seeing partial throttle, and 55 Hz, then I need more pull on the spring with the same change in stretch. The only way to get that is to reduce the spring constant (but readjust the zero point preload) so that the change in pull for any change in distance the spring is stretched is less. That means to reduce the spring constant, which can be done either by actually changing it, or by changing R as per the manual.

If you change R, you change both the distance from the governor axis that the spring works on (changes the torque even if the force of the spring were unchanged) and the distance the attachment point for the spring moves towards the spring. That's why the formula above has an R squared term.

OK, so now to my problem. I'm at the minimum R. The spring has hit the governor shaft. I can't reduce R any more. I tried adding a longer spring and lower spring constant. That worked great, (a bit sensitive in hunting, but not too bad) but that spring won't fit in the space available.

And now my second question. The manual says that the failure to be able to adjust could be caused by the spring weakening. I think of a "weak" spring as one that has a reduced spring constant. I wouldn't think that buying a new spring (with a stronger spring constant?) would help me. If the torque is correct at no load angle1 and 62 Hz, it will drop off even faster at angle 2. Is it possible that the newer spring has a weaker spring constant?

I know the formulas above are approximations. I've left off various constants of proportionality and used the small angle approximation, which may not fully apply. That's why I'd like to read a description of operation or hear the thought of others.

Thanks for any comments.
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  #2  
Old 02-28-2012, 01:39:39 PM
Jim Rankin Jim Rankin is offline
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Default Re: Mechanical governors, torque and springs

I think about the adjustment of the working radius of the spring on the governor arm as more of a sensitivity adjustment to stop hunting, but I guess it also makes the throttle response to any decrease in rpm be greater.

What I think has likely happened is the spring has stretched so it's pull drops off too fast as it moves the throttle open. It responds, but runs out of strength/travel. Actually increasing R of the spring attachment is going to make it's pull be more effective, but at the expense of the distance it can move the governor arm.

Order a new spring if it's available

If new is not available, find a spring with more turns of a slightly thinner wire.
Think of the difference between a screen door spring and a short thick spring. The long spring has a lot more travel before it breaks or is permanently deformed compared to the short spring.

Alternatively you may need to decrease the effective radius of the throttle arm on the carburetor or increase it on the governor arm so you get the full travel you need. With the engine stopped, does the throttle go full open or not?
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Old 02-28-2012, 04:44:49 PM
Magnetite Magnetite is offline
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Default Re: Mechanical governors, torque and springs

Quote:
I think about the adjustment of the working radius of the spring on the governor arm as more of a sensitivity adjustment to stop hunting, but I guess it also makes the throttle response to any decrease in rpm be greater.
It's both. As you decrease the working radius, you are making it so that a smaller change of RPM makes a larger change in the position of the governor.

Quote:
What I think has likely happened is the spring has stretched so it's pull drops off too fast as it moves the throttle open.
A pull that drops off faster would be a larger spring constant. Assume I stretch a new spring one inch and it pulls with 10 pounds of force. Over one inch, it would go from zero to 10. That's a spring constant of 10 pounds per inch.

If that spring ages, I'd expect the spring to go from 10 pounds to say 9 pounds. Over the same inch, it would drop from 9 pounds to 0. That's a spring constant of 9 or a decrease. That's a decrease of spring constant.

If I add a bit of stretch to get it back to 10 pounds (with the speed adjustment), then over the same one inch, it would only drop from 10 to 1 pound. That's what I want, more pull as it moves. I admit, I don't know if a spring constant increases or decreases with age, but I find it hard to see how the pull of 10 pounds could increase in any way.

Quote:
It responds, but runs out of strength/travel.
A smaller spring constant runs out of pull more slowly with distance than a larger spring constant.

Quote:
Actually increasing R of the spring attachment is going to make it's pull be more effective, but at the expense of the distance it can move the governor arm.
Exactly. That's why I gave the formulas. Changing R has two effects. The distance the spring moves is roughly angle times R. The force from the spring is that distance times the spring constant. OTOH, the torque is force times R. Changing R is like changing the spring constant

Quote:
Order a new spring if it's available
Why? Yes, I know that's what the manual says. I also know that replacing the spring solved my hunting problems on my 4.0CCK. But part of the fun is understanding what's going on. If the new spring is stronger than the old spring for the same stretch, then it's spring constant is greater, but my understanding is that I need a lower spring constant. I'd hate to buy a new spring and find that it had a larger spring constant.

Either my understanding is wrong, in which case I'd like to understand better, or the new spring will have a lower spring constant, in which case I'll be surprised. Either way, I'd like to understand it.

Quote:
If new is not available, find a spring with more turns of a slightly thinner wire.
I have dozens of springs, and you are right, more turns of smaller wire will give a lower spring constant (which I think I need). In fact, that's exactly what I successfully tried - I used a longer spring with more turns and thinner wire. I attached it at a greater R and it worked well, but I had to find a new attachment point at the far end and the speed adjustment didn't work (too close). This confirms that you think a new spring will have a lower spring constant, which seems surprising to me. Perhaps my spring has become non-linear - it no longer has a uniform spring constant over the stretch range?

Quote:
Alternatively you may need to decrease the effective radius of the throttle arm on the carburetor or increase it on the governor arm
This was my next thought - increase the length of the governor arm. The throttle arm was originally damaged by the previous owner. Perhaps it's been lengthened enough by that damage to produce this effect. It was badly bent up and completely loose on the carburetor shaft, which was also slightly bent.

Quote:
With the engine stopped, does the throttle go full open or not?
Yes. That's the first adjustment and I've checked it twice. It's full throttle with engine off I'm really wondering about the carburetor attachment point now. I think lengthening the governor arm would be the easiest, but the problem may be related to the original damage to the carburetor.

Thank you for the comments.
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Old 02-28-2012, 05:50:52 PM
RSCurtis RSCurtis is offline
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Default Re: Mechanical governors, torque and springs

Before you start changing parts, adjust the relationship between the governor arm and the carburetor so that the carb. butterfly is almost wide open when the engine is not running. It should not be against its stop. This will increase the sensitivity of the governor. Second, move the spring as close to the governor shaft as possible, and increase its tension to maintain frequency. Onan's typically won't run wide open at full load, as they are typically overpowered. The reserve throttle is available to pick up transient loads. My 4000 watt OF6 with a 2500 watt load on it runs with the throttle barely open, and maintains less than one cycle difference between 0 and 2500 watts.
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Old 02-28-2012, 06:22:34 PM
bbuchorn bbuchorn is offline
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Default Re: Mechanical governors, torque and springs

? You say that the throttle on your NH isn't all the way open under full load? How is it loaded? My 6.5NHE runs fully loaded by amps -6500 measured.
at about 3/4 throttle. this gives me a little surge value which is great for starting motors and such. Just adjust the governor to give you the proper frequency value at unloaded throttle, about 61 or 62 HZ then load and adjust the sensitivity at various loads keeping the frequency in the desired range, 58-62 works fine. Having throttle left after fully loading your generator tells you that the engine is running really well. Take good care of it and count your blessings. Mine had an internal problem with the governor assembly that caused the rpm to rev up when it got hot. took forever to figure that the nylon ball spacer pressed onto the cam gear was slipping when it got hot.
Good luck with your rig. BB
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  #6  
Old 02-29-2012, 10:52:56 AM
Magnetite Magnetite is offline
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Default Re: Mechanical governors, torque and springs

Quote:
Originally Posted by RSCurtis View Post
Before you start changing parts, adjust the relationship between the governor arm and the carburetor so that the carb. butterfly is almost wide open when the engine is not running. It should not be against its stop.
Let me make sure I understand this: You are suggesting I shorten the rod slightly so that it can't reach full open throttle? It is currently set per the manual - the rod is set to touch the full throttle, full open stop with the engine off.

Quote:
This will increase the sensitivity of the governor.
Can you explain how? Is it because it will change the part of the arc of motion of the connection point between the governor rod and the arm of the carburetor that the governor rod connects to? As I hinted above - when I got the gen, the arm on the carb was loose on the throttle shaft. I could rotate the arm about forty degrees without changing the throttle plate. I didn't have a good reference to fix it, so I chose what seemed to be the best and closest to original location for the throttle shaft arm, but perhaps I need to rotate the arm on that shaft to a new location? A description of the exact angle between the arm connection point for the governor rod and the throttle plate would let me check that, but I haven't found any pictures or specs that would give me that angle.

Quote:
Second, move the spring as close to the governor shaft as possible
That's where it is now - against the governor shaft.

Quote:
and increase its tension to maintain frequency.
Increasing tension will increase RPM. I can set any RPM I want, for a given load, but if the load changes the RPM and throttle plate position aren't correct (not within the spec of 4 RPM droop between no load and full load)

Quote:
Onan's typically won't run wide open at full load, as they are typically overpowered. The reserve throttle is available to pick up transient loads. My 4000 watt OF6 with a 2500 watt load on it runs with the throttle barely open, and maintains less than one cycle difference between 0 and 2500 watts.
I've measured my resistive load bank elements at different voltages and know that my gen will produce a bit more than the rated 6500 Watts. The problem is that I can't get to within the specified droop, even with the spring against the governor shaft. I have enough throttle left to get to within the specified range, and that's my dilemma. Buying a new spring would seem to me to go the wrong direction (increased spring constant when I need a decreased spring constant) but I can't be sure, so I wanted to check that there isn't some other problem before buying something I don't think I need.

I thought about the carburetor arm and where it is rotationally positioned on the throttle plate shaft before posting here. It is possible that my arm is in the wrong place, since I had to fix it when I first got the gen. However, at full throttle, the rod connection point on that arm is on the far quadrant away from the governor. It seems to me that that location is the most sensitive position. A small increase in the push of the rod should cause a larger rotation in that quadrant than if I moved the arm to the quadrant closer to the top of the shaft and closer to the governor. (that may not be very clear - if it's not, let me know).

---------- Post added at 09:52 AM ---------- Previous post was at 09:29 AM ----------

Quote:
Originally Posted by bbuchorn View Post
? You say that the throttle on your NH isn't all the way open under full load? How is it loaded? My 6.5NHE runs fully loaded by amps -6500 measured.
What voltage and RPM do you have at 0 load and at full load? I have it loaded with resistive loads, and have corrected wattage for the voltage droop. If I have it adjusted for minimum droop, I still have 6.5 RPM droop, not the specified 4 RPM, but if I manually twist the throttle open, while at full load, I can easily get to within 4 RPM (or less). I just need the governor to provide a bit more throttle. I can get that by changing the spring to a lower spring constant, but the spring that works won't mount in the available space (too long and won't connect at the end to the speed adjustment rod.

Quote:
Just adjust the governor to give you the proper frequency value at unloaded throttle, about 61 or 62 HZ then load
I've done this - no porblem.
Quote:
and adjust the sensitivity at various loads keeping the frequency in the desired range, 58-62 works fine.
I can't. The droop is so much at one end of the adjustment that the VR cuts off the output voltage (protection feature for frequency sensitive devices). At the other end - most sensitive next to the governor shaft, the drop is 6.5 Hz (I realize I said 6.5 RPM above - that was an error - it was 6.5 Hz. droop.)

Quote:
Having throttle left after fully loading your generator tells you that the engine is running really well.
Yes. I'm not worried about extra throttle. I'm worried that I can't get the governor to tell the engine to turn faster when it clearly has enough power to do so and the manual says I should adjust it to turn faster at full load.

Quote:
Take good care of it and count your blessings. Mine had an internal problem with the governor assembly that caused the rpm to rev up when it got hot. took forever to figure that the nylon ball spacer pressed onto the cam gear was slipping when it got hot.
I haven't opened the governor, and with luck, I won't have to. There are several solutions to my problem. I could lengthen the governor arm. I could shorten the carburetor arm. I could change the mounting position for the spring. However, all of those take me away form how it was designed to work, and I'd like to avoid that. Something isn't right, and I'd rather find that something and fix it instead of making a hack that counteracts the problem.

Would you (or anyone else with an NHD, NHE or one of the similar BG series generators) measure or take a picture of the throttle plate arm when the throttle plate fully open (throttle plate level and aligned with the axis of fuel/air flow through the carb? I need to know the exact angle between the arm on the throttle plate shaft (the arm that connects to the governor rod) and the throttle plate. Since my throttle shaft was bent and the arm was loose on that shaft, perhaps my problem is there? It might be a simple matter of confirming that the arm is parallel to the throttle plate.

Thanks to all who have taken the time to respond here.
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  #7  
Old 02-29-2012, 11:01:56 AM
pegasuspinto pegasuspinto is offline
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Default Re: Onan Mechanical governors, torque and springs

Ohy, all that math makes my brain itch! lol

Think of it this way-if the spring is too weak, it's too easy for the governor to close the throttle, if you took the spring off, the governor would likely instantly put the set at idle.

One other thing to do, is to nudge the throttle open at full load. If you can't manually run it back up to 62hz (and beyond) you're outta horses for some reason.

ANOTHER thing, is generally when I get where you are at, I would study the throttle linkage and try to make sure the rod and spring are back in OEM position (looking at the wear patterns), sometimes everything suddenly works perfect if you reverse illicit tunerey attempts.

Robert
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Old 02-29-2012, 11:35:07 AM
Magnetite Magnetite is offline
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Default Re: Onan Mechanical governors, torque and springs

Quote:
Originally Posted by pegasuspinto View Post
Ohy, all that math makes my brain itch! lol
I spent a while trying to just think it through, but eventually decided I had to use some math to get a decent understanding (not that I'm there yet )

Quote:
Think of it this way-if the spring is too weak, it's too easy for the governor to close the throttle, if you took the spring off, the governor would likely instantly put the set at idle.
Yes, but no matter how strong or weak the spring is, you can always pull on it hard enough to overcome the governor (at any single speed) by adjusting the speed screw (which just pulls harder on the spring). We're not concerned with pull at one speed. I can set the speed correctly at any load. We're concerned with how the spring pull changes, and what we need is less change. Right now the spring force falls off too quickly. Conversely, it increases too fast if I set the right speed at full load. I need a spring that doesn't change that fast. I generally call that a "weaker" spring. My car spring goes from zero to a thousand pounds of force in an inch. My generator spring goes from zero to ten pounds or so in the same inch. That's a weaker spring, but it's still too strong, unless I'm missing something.

Quote:
One other thing to do, is to nudge the throttle open at full load. If you can't manually run it back up to 62hz (and beyond) you're outta horses for some reason.
Exactly! That's why I said I still have some leftover throttle. With the engine at full load, I have plenty of throttle left over to get it back to 60 Hz or more, but the governor has it at 55 Hz. I just need to get the governor to ask form more throttle and figure out why it isn't doing that now.

Quote:
ANOTHER thing, is generally when I get where you are at, I would study the throttle linkage and try to make sure the rod and spring are back in OEM position (looking at the wear patterns), sometimes everything suddenly works perfect if you reverse illicit tunerey attempts.
Exactly! (again ) I've tried resetting to default several times. I've double checked that there's no binding, that the rod looks like the rod in the manual, etc. I'm concerned that the position of the arm on the throttle shaft may be wrong (see above), but I have no way to be sure of where it's supposed to be. My best guess is where it is now, and in thinking about it, I suspect that any other position will make things worse, not better. Still, something is cockeyed.

---------- Post added at 10:35 AM ---------- Previous post was at 10:25 AM ----------

[QUOTE=RSCurtis;743221]adjust the relationship between the governor arm and the carburetor so that the carb. butterfly is almost wide open when the engine is not running. It should not be against its stop. This will increase the sensitivity of the governor. /QUOTE]

I've been thinking about this comment, and it makes a lot of sense. I may even be able to compensate slightly for the incorrect position of the arm on the throttle shaft (if it is wrong). Thank you. I'll try this next.
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Old 02-29-2012, 11:56:53 AM
RSCurtis RSCurtis is offline
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Default Re: Onan Mechanical governors, torque and springs

That should solve your problem, and it also eliminates stress on the bell crank when the governor reaches the stop on the carburetor. The purpose of doing this is to start the throttle at the beginning of the governor's travel. If this relationship is sufficiently deviated from, the governor runs out of travel before the throttle is closed enough- hence your higher than desired unloaded speed.
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Old 02-29-2012, 01:10:52 PM
pegasuspinto pegasuspinto is offline
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Default Re: Onan Mechanical governors, torque and springs

the governer response is non-linear as well, a balance of forces is important to keep everything happy, given what you said, i'd first move the spring to a hole further from the shaft, and re-adjust. next would be a new spring, and one other option would be to add a spring-it could be set up fairly loose at WOT and then come into play quickly, making the combo of the two non-linear...

Robert
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