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Oil


 
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Minirob
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PostPosted: Tue May 14, 2019 20:53    Post subject: Oil Reply with quote

Hi

Just a general question.

If I slowly turn a component or toothed sprocket in contact with oil, obviously it gets coated in oil. The oil can be seen running off.  Is there some sort of point at which oil no longer lubricates due to the rotational speed of the component or geared sprocket?
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Simon_W
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PostPosted: Tue May 14, 2019 21:33    Post subject: Reply with quote

There probably is, yes. That's why oil is constantly pumped, sprayed, and circulated within the engine. So there's always a good supply of the stuff.
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assassin
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PostPosted: Sun May 19, 2019 3:09    Post subject: Reply with quote

Yes its called its lubrication efficiency yield point and this is the point at which it begins to fail lubricating as it should.
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Simon_W
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PostPosted: Tue May 21, 2019 23:04    Post subject: Reply with quote

assassin wrote:
Yes its called its lubrication efficiency yield point and this is the point at which it begins to fail lubricating as it should.

I'm always interested in what you write Assassin. I am wondering why the phrase "lubrication efficiency yield point" produces no search results.  I can find references to yield point and yield stress in greases, but that's the closest I can get.
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assassin
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PostPosted: Wed May 22, 2019 3:18    Post subject: Reply with quote

Because its a generic engineering term to define a specific point in lubricity, unless you know and understand it its unlikely you would have heard of it or be able to easily find it.
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Minirob
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PostPosted: Wed May 22, 2019 6:22    Post subject: Reply with quote

Yes Simon_W, totally agree. So if thickies like me keep asking basic questions, we will be forevermore enlightened  Very Happy
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Simon_W
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PostPosted: Wed May 22, 2019 23:19    Post subject: Reply with quote

assassin wrote:
Because its a generic engineering term to define a specific point in lubricity, unless you know and understand it its unlikely you would have heard of it or be able to easily find it.

If it's a 'generic engineering term' then surely I should be able to find several examples of its use online?  This would be unrelated to whether I know and understand it or not.  

Taking your logic, as you know and understand this term could you please find me an example of its use?
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assassin
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PostPosted: Mon Jun 03, 2019 1:47    Post subject: Reply with quote

Not easily as this is predominantly testing data from machine companies or lubricants companies and they rarely release this data these days.

If you can find the old Radicon gearbox site they used to put it in their technical data section.
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assassin
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PostPosted: Tue Jun 04, 2019 4:25    Post subject: Reply with quote

I will explain it simply just for those interested.

When you design a gearbox there are many factors to consider and I will do this in reverse order at it is easier to explain.

You usually begin with two speeds and these are the tangential and rotational speeds and the tangential speed is the speed that something comes off at an angle of 90 degrees to the rotation; this dictates the type of lubricant used as a low speed can use grease and medium to high speeds can use splash or a pumped feed to the gears as these dictate the speed and quantity of lubricant delivered and if this is sufficient for its use.

Next we have to consider coefficient of friction and the rolling and sliding friction as these  are the ramp up loads when gears are engaging, the full load when the gears are fully engaged, and the ramp down load when they begin to disengage and the next tooth is fully engaged and the rolling friction is when a tooth is engaging and disengaging and sliding friction is when a tooth is fully engaged and this dictates the clearance created by the film of oil between them.

Next is cooling and the lubricant has to sufficiently cool the gears in contact and transfer the heat away from the gears and absorb it into the lubricant and from the lubricant to either ambient air, water, or through a cooler to dissipate the heat.

Next is relative motion, is the component a low speed, medium speed, or high speed item and is it constantly moving or intermittently moving? a gear in an automotive gearbox is constantly moving and needs oil as a lubricant and to dissipate the heat through the gearbox casing as air passes it to remove the heat and maintain temperature. In a steering rack ball joint this is intermittent movement as you only move the joint when you turn the steering and you use grease for these and a grease reservoir which is the rubber boot which holds a small reservoir of grease. If you have two gears meshed in a manual gearbox you can use a splash feed in one of two ways, if the speed is sufficiently high you can use a sump where the lower gear turns sufficiently fast to pick up oil from its immersion and create fling, this is where the lower gear turns sufficiently fast enough to pick up oil and throw it around to lubricate itself and splash its meshed gear so both gears are lubricated before they actually mesh, in a low speed gear system the lower gear will have to turn before it picks up oil and transfers it to the meshing gear if the speed is too low to throw the oil.

In an automatic transmission the heat range is excessive compared to a manual gearbox and the ATF acts as both hydraulic oil in the valve chest and is compressed as a hydraulic fluid and as a gear lubricant in the epicyclic gear clusters as well as a lubricant for the brake bands or friction plates and friction plates and compression as hydraulic oil raise working temperatures well outside its kinematic range which usually means an oil cooler is added to control this temperature an as it is released it sprays above the gear sets instead of returning straight to the sump.

These factors dictate the lubricity efficiency yield point and this is the minimum point at which the lubricant does its job efficiently and provides efficient lubrication for its application, this dictates the type of lubricant to be used, the speeds or types of gears to be used along with their teeth size, the configuration of the teeth such as straight cut or helical gears as helical gears throw lubricant at a wider angle, and the quantity and depth of lubricant required in the sump to efficiently lubricate the gearbox, and even the type of material of the casing to disperse heat to maintain its kinematic viscosity.
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