Lubrication the myths, lies and the truth

Lubrication is a hot topic in the laser community, it is very rare that two people will agree on the correct type of lubrication to use. Thankfully very few components require lubrication due to the stepper motors and idle pulleys utilizing sealed bearings.

The bearing carriages on the X and Y axis require lubrication as they are not of a sealed type due to the design featuring ball bearings in direct contact with the linear rail. Due to the type of bearing and the atmosphere, they are exposed to they require regular lubrication to ensure a long working life.

I have posted a screenshot below of a question I must see asked once per week. The replies are typically the same, although this was the first time I have seen a 9b graphite pencil specifically required .

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Bearing construction

MGNC BEARING CONSTRUCTIONMGN12C linear bearing
MGN15C Linear bearing

A grease nipple is available for MGN15 bearings for lubrication with a conventional grease gun, The MGN12C bearing commonly found on the bulk of our machines can be lubricated via the hole at the side of the end cap.

Friction

The coefficient of friction for a linear guideway can be as little as 1/50th of a traditional linear slide. When a load is 10% or less than the basic static load rate, most of the resistance comes from the grease viscosity and frictional resistance between balls. In contrast, if the load is more than the basic static load rating, the resistance will mainly come from the load. This statement basically dismisses the theory that using grease vs oil will cause diminishing results, sure it will slightly increase the torque required to move the slide, but the majority of the resistance is coming from the load anyhow.

Should I use grease or oil?

Neither is incorrect both oils and greases are recommended by OEM manufacture of linear bearings. But they both have there recommended use cases.
For speeds over 60 m/min, High viscosity oil should be used as a lubricant. For speeds under 60 m/min Lithium soap-based grease is recommended. 60 m/min = 1000mm/s

Please take into consideration when using oil, linear slides are designed to have a continuous oil flow via a central oil distribution system, Hiwin recommends an oil feed rate of approximately 0.3cm^3/hr. Hiwin also recommends that oil is pumped into the bearing via the lubrication port. Not applied to the rails or sprayed into the bearing sides.

Lubrication frequency

Hiwin recommend that linear guideways must be lubricated before installation, and re-lubricated every 100km
For high viscosity oil-based lubrication a flow rate is recommended of approximately 0.3cm^3/hr
You can always calculate the exact oil-based lubrication required by using the following formula.

Oil based lubrication frequancy for linier slides

Bearing key performance indicators

Monitoring the bearings telltale factors is a great indication of the health of your bearing and the effects using the correct lubrication will have.

  • Bearing vibration –  Using grease of the incorrect viscosity will lead to excess vibrations from the bearing, this is transmitted through the optical system and can become apparent in the quality of your engravings, especially at higher speeds.
  • Bearing noise – Low viscosity oils will cause the bearings to become audible. You shouldn’t be able to hear a properly lubricated linear slide.
  • Bearing temperature – if your bearing is hot to the touch, its to late. Swap your bearings out and use the correct lubrication.

The effects of incorrect lubrication

Using the incorrect lubrication type will lead to premature bearing failure, or worst case fretting of the surface of the linear rail.
Incorrect application of lubrication i.e by wiping the rails will increase particle ingress into the bearing causing increased friction, heat & uneven distribution of lubrication.
Some people claim that by using grease vs oil will increase the ingress of dirt/particles into the bearing itself. This is not the case. A correctly lubricated bearing will expel all previous grease when recharged, the research and development of the lubrication industry is nothing short of amazing. A well-designed lubricant will suspend particle deposits and prevent them from adhering to any contact surfaces.

Fretting of an NSK linear slide

Example of fretting of an NSK linear rail

How to apply lubricant

For MGN12C bearings with no grease nipple, the following grease gun is required. Any pump-action grease gun with a pointed nozzle will be sufficient. Just press the nozzle into the endcap hole and apply a few pumps of fresh grease. Wipe away the excess grease that escapes from the bearing.

pump action grease gun

For MGN15C bearings with grease nipple, the following grease gun is required. Any pump-action grease gun with a nipple nozzle will be sufficient. Just press the nozzle onto the grease nipple and apply a few pumps of fresh grease. Wipe away the excess grease that escapes from the bearing.

pump action grease gun

Conclusion

Always follow the manufactures guidance and instructions when performing maintenance. Using lubrication of the incorrect type can cause more issues long term than using no lubrication.

After observing the recomendations in the manufactures guidenotes. Found in this pdf on hiwins website.  https://www.hiwin.com/pdf/linear_guideways.pdf 

We recommend the use of a lithium-based soap grease based on the following specs.

 

Recomended grease

The most suitable grease we have found available is SKF LEGE 2 https://www.skf.com/uk/products/lubrication-solutions/lubricants/high-performance-low-friction-grease/index.html

It is hard to recommend a lubrication frequency, but based on hiwins 100km value if we use the following calculation as a rough guide on what 100km equates to in laser head travel.

If you were to engrave 700mm width x 500mm height with a 0.1 scan interval the X-axis would cover 3.5km, the Y-axis would only cover 700mm (0.0007km) This value and calculation is only to give you an idea of distance covered during engraving operations.

A better way to calculate lubrication frequency might perhaps be time.
Let’s say you operate your machine for 6 hours a day with 80% utilization, engraving at 400mm/s
This would equate to a maximum distance traveled of 6.912km per day. This would dictate lubrication being required every 14 working days (for the X-axis)

To calculate your X-axis working distance use the following formula

Speed (S) mm/s

Utilization (U) %

Working hours (W) h

Distance traveled in km =((W*U/100)*60*60)*(S/1000000)

This is only an approximation, performing greasing at shorter more frequent intervals is never a bad thing.

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