Lubricant for GEM equatorial mounts

Back to basics first, some of background info

Penetration 針入度 

Penetration 針入度 係指標準圓錐體在5秒鐘內，沉入潤滑脂

dN value ( more applicable for oil)

dN would equate to [½] x [OD+ID] x rpm  

average diameter is 60mm

rpm is 0.1

so dN is 6 nominal

components of  lubricating grease:

Dropping point (滴點) is generally used for greases, and indicates heat resistance of a grease up to a temperature. At this point, greases tends to start dropping down a vertical surface. Grease does not remain much useful beyond this temperature. 

The formula for kinematic viscosity is 1 centistoke (cSt) equals 1 millimeter squared per second (mm2/s). This is a rate of flow. It is the time it takes to have a known amount of fluid flow a given distance. There is no external force pushing the fluid. Only gravity is used. This means the fluid’s weight or density helps it to flow. Kinematic viscosity incorporates fluid density as part of its measurement.

Types of grease : Valvoline 101

pros and cons of synthetic oil base 

difference types of thickeners:

Most high performance lubricant emphasizes hi temp operation. However, for amateur equatorial mounts, just need normal temperature range, and sometimes snowing conditions (< -20 deg c)

 

Most high performance lubricant emphasizes high rotation speed, for amateur astronomers, extremely slow

The gearboxes of mounts are not hermetically sealed or airtight, lubrication oil will leak and cannot be used. So only grease can be used

The main purpose is to reduce friction, reduce noise, reduce vibration, thus aiming to achieve better tracking and lower periodic error

do a PE guiding error test (max, rms, trend), to see how good/bad your existing tracking is. So you can compare after your maintenance / upgrade job.

Degrease thoroughly, before changing the lubricant. you cannot tell which lubricant was used just by color or appearance

IMHO, proper meshing/alignment of gears are more important than the lubricant used 

Equatorial mounts are normally well balanced by counterweight, so torque required is minimal. Although load is moderately high ( 10 to 70 kg load nom, 2 to 14 Nm), 

extreme pressure (EP) grease is not a must. (  eg Aeroshell 64, Tungsten Disulphide WS2 powder, and other moly added)

Rather,  smooth operation is more preferred, or low stiction thus NLGI of 1 or 2 preferred 

Starting torque is less of a concern. Slow starting does not affect observation at all, and you rarely need to start and stop the tracking many times during the whole night.

For worm gear lubrication:

Most preferred from various forums:lubriplate 105,  Loctite super lube multi purpose, PTFE thickener, NLGI 2 or 1(preferred)

 

Loctite silicone lubricating grease is NOT for worm gear lubrication. It's mainly for o-rings, or waterproofing. It's mineral oil base

Lubriplate 105 or (parktool PPL-1) is the long time standard choice, but it is using mineral oil as base oil. Not suitable for sub -20 deg c environments

There's nothing wrong with lithium based lubricant. You just need to know what type of base oil and thickener is used, and also operation temperature. Many commonly( G/T7324-2010) found has a narrow operating -20 to 120 deg c, and thicker than NLGI 2.

However, Mobilith SHC 220 is a good lithium complex grease

Calcium complex, calcium sulphonate,  thickener type is also good candidate, eg Morris K90, or AnRunt ANR 20G. Some say even better than Li based

Molybdenum (Moly), or molybdenum disulphide (MoS2) is additive for extreme pressure, and thus overkill for this application, eg Mobil Aeroshell 64 (formerly know as 33 Ms) (lithium based) 

how about bicycle grease?

Beware for using bicycle lubricants, lubricating oil should be ruled out

Finishline grease  is synthetics with teflon , NLGI-2 and other EP additives, ( very good on advertising), should be very similar to super lube, or Krytox GPL 205

Nye Neogel 760G, synthetic , PAO and Silica, NLGI 2, op temp -40 -125 deg c.

Some of usual complaints: dried out, oil separation:

When used longer than lifetime/durability specified, any grease will dry out

When used outside the operation temperature, or due to oxidation, any grease will separate

Other good candidate:

 

Celestron SCT baffle tube: 

- Dow Corning vacuum grease, for high stickiness

Helical Focuser:

- polyurea, NLGI 1 or 2

Strain wave gear :

yellow SK-1A grease 

Machine design 

Caltex 

dynaroll 

machinerylubrication 

NSK Lithium complex grease patent 

Mobil grease selection guide

mfgrease vg cc fd 2/400 is NSF H1 food grade, good for coffee grinder, food blender