G11 slipping??? 2001ish
Posted 02 August 2012 - 12:54 AM
Last weekend I wend to put away the scope on the mount and notice that no matter how hard I crank down on the RA clutch I still can freely move the scope. Dec is fully locked.
Do I need to service something? or get new clutch pads?
Posted 02 August 2012 - 01:06 AM
Easy to clean, too. Unscrew the RA clutch completely, remove the shaft (note the arrangement of the washers as they come out). Clean the nylon pad - both sides - with a household degreaser like 409.
If that doesn't work, then the pads might be too polished to grip and then it's a case of either roughing them up with some sand paper or getting new pads.
Posted 02 August 2012 - 12:21 PM
Sounds like you probably need to clean the pads. Oil can migrate onto the pad surfaces and if the mount spent any length of time on its side, there's a good possibility that the pads were compromised.
I do not wish to offend but this is not so.
There are two major clutch designs on the market these days. One, familiar to Vixen, CG5, Tak, and old AP users, involves the use of lock lever clutches. When you lock the lever down the mount cannot be moved *except* by an electronic paddle *or* by unlocking the clutches.
The slip clutch design in the 90s and later AP mounts and in the G8 and G11 is meant to give you total control of the scope either by pushing by hand or by using a paddle (pushing by hand will however wreck the pointing model/catalog alignement of a gemini or an AP mount). So if you want to adjust using the paddle, do that. If you want to adjust by hand, do that.
The clutches are more than adequate to hold a balanced tube even with heavy photographic equipment and counterweights. That is, the mount will hold and track, resist wind, and it will, when very tight, manifest a physical resistance to being grabbed and moved.
That said, the amount of leverage you have by grabbing the end of the telescope tube or the end of the dec shaft (for example) is so great that no matter how tight you make the clutches you will be able to "break" the clutch hold and move the scope.
This is the way it is supposed to be. No matter how tight you make the clutches you will not interfere in the scope's ability to slew (manually on a stepper g11, or electronically on Gemini, and ditto for AP mounts QMD through GTO).
Once every year or two here or on Losmandy we see people posting their expectation that a slip clutch mount can be totally locked against all force. That is a misconception. The design makes it impossible to turn the mount against the worm gears. If I recall correctly, that is not true of a lock-lever mount. If you turn against a lock lever, which will be difficult, you will likely damage the worm or worm gear ring.
If you have the clutches tight and the mount is not tracking, there could be (a) serious imbalance or (b) the mount is hitting something or © a very rare issue, such as the Oldham coupler is not engaged.
On a Losmandy stepper motor mount (and the old APs) the stepper motors' intrinsic function is to keep on chugging. If the mount hits itself or bumps the tube against a tripod leg, the stepper will keep turning and the clutch will slip and nothing will happen. In some kind of extreme case, the stepper motor will stop. No harm no foul. Steppers do not burn out. They wait for you to make the problem go away.
Servo motors by contrast can burn out and so control paddles will have feedback loops to shut down power when the slewing activity of the mount gets erratic.
In a servo system it is actually possible to have the clutches not quite tight and then the inertia of starting up a particularly heavy load might cause clutch slippage. This can be avoided or reduced by changing slewing speeds, and if I remember right the computer also calculates the normal slippage and can compensate for it.
And thus much on clutches.
p.s. I will add--having made this mistake when I got my G11--that a common clutch illusion is, with no tube on the mount, to push on the edge of the saddle on top of the dec axis and observe that it can't be moved. That is because you are two inches from the axis of rotation and your index finger and even your hand is not going to exert enough force to "break the clutch hold." Say you're exerting twenty lbs force at two inches, that's a 40 lb load, the clutch says I can hold this no sweat.
By contrast, if you grab the end of the counterweight shaft and give it a yank against a completely tight clutch you are now perhaps pulling with twenty or thirty pounds of muscle force on a lever that is maybe 30 inches from the axis of rotation. With 900 inch pounds of force exerted the RA axis will turn quite easily. But this has not much to do with how the mount will hold a 50 lb payload and 40 lbs of counterweights, when the clutch is in essence holding against zero effective leveraged weight.
Posted 02 August 2012 - 02:26 PM
Posted 02 August 2012 - 03:59 PM
Posted 02 August 2012 - 04:18 PM
I am talking about 1-2 grams of force applied to the 12 lbs ota on a well balanced mount. I don't remember it being this easy to move, nor was it the first few nights. Thank you for the tomb of info on the clutches.
Now that we exclude newbie errors, consider getting UHMW pads from Losmandy. Clean the pad surfaces and the areas where they seat with alcohol to remove all grease: the G11 clutches run "dry," you don't even want finger oils. Your mount is at the borderline age where they might still have the nylon clutches. Nylon is solid white, UHMW is semi translucent cream.
You might also want to take a look at the order of washer placement on your clutches, you should find some diagrams pics in files on Losmandy Yahoo.
I am unaware of anything that would cause these pads to wear out. It has not been reported, with the exception of leather clutch pads used in the Titan (don't know if they are still using leather).
Posted 02 August 2012 - 06:24 PM
Still tracks perfectly for being stored for 9 years
Hmm. Could be that the grease has separated and oil has migrated to the plastic discs. Certain Lithium greases can separate over time and 9 years is certainly long enough. If you haven't done so, it's time to take the thing apart clean and regrease. PLEASE, LET'S NOT START ANOTHER GREASE THREAD.
The plastic clutch discs must be dry with no oil or grease on them. Same goes for the Aluminum discs that do the clamping.
A synthetic grease usually will not separate. There are grease threads in this forum. I suggest reading up on them to find out what a powderkeg of a topic grease can be.
But I still think you need to clean and regrease. When you have it apart, if the old grease is white, it's probably lithium. If it is, check the roller bearings to see if the little rollers rotate freely in the cages. If they don't then you should clean out the old grease and residue on them and regrease the bearings. Don't use too much, it only takes a little bit. Report back what you find out.
Posted 02 August 2012 - 06:47 PM
Posted 03 August 2012 - 07:55 AM
the clutches just do not have the grip to support the refractor that is not perfectly balanced.
For me this is the key. If you balance the scope properly, you won't need to crank down on the clutch knobs. My G-9 is 17 years old, works lovely and I rarely clean the nylon disks.
Posted 03 August 2012 - 05:07 PM
Ordered pads. Thanks much!
Posted 06 August 2012 - 02:52 PM
Posted 06 August 2012 - 04:23 PM
Here is a question for you...are you using a polar-alignment scope? I have a G11 that I bought used, so I'm not sure of its vintage. I had the problem you describe; a polar axis that could not be tightened effectively. In my case it turned out to be the polar scope preventing the clutch knob from doing its job. If I didn't use the polar scope, the clutch would tighten just fine. I figured this out after buying the new clutch pads.
If you had to buy UHMW that means it predates 2001. If it says Celestron or has two altitude lock screws that puts it in the first half of the nineties. If it has Hurst stepper motors (which should be replaced) that means it can be no newer than 2001-2.
Posted 13 December 2012 - 03:38 PM
If you should feel the need, the serial # is usually on the bottom of the mount. This can be looked up by Losmandy to give the vintage of your mount. That said, it sounds like your clutch pads aren't holding. Since the mount has been stored for 9 yrs it might be prudent to do a clean & regrease with lubricant of your personal choice. The G11 is designed to be dead simple to work on. You'll need a good set of allen wrenches (both ASE & Metric), feeler gauges, and screwdrivers. (I use a set of hex headed insert type). Another source of excellent info is the Losmandy Yahoo group.
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