Oil flinger/labyrinth combination question 5

[MechatroPro]

i have this drawing of a gearbox, which includes a labyrinth type side cover(to seal the shaft) and an oil flinger(i assume to help lubricate the components inside the housing? ) next to it, however i don't understand how this setup works to seal the shaft and lubricate components, and the required fit between the shaft and oil flinger.
any hits about it is appreciated.

 

[MechatroPro]

i forgot to attack the drawing, here it is:

http://oi60.tinypic.com/111rx2x.jpg

 

[tbuelna]

Unfortunately you did not provide a sketch of your gearbox seal.

But based on my experience with similar devices, here's what I'd speculate: First, the primary purpose of the slinger is probably not to lubricate the components in the gearbox. Instead, the primary purpose of the slinger is probably to deflect oil away from the labyrinth seal. Second, the labyrinth seals usually prevent oil leakage in one of two ways- using a pressure differential, or using a pumping effect. Some lab seals use compressed air flow fed into the labyrinth spaces to push oil back into the housing. Other lab seals use a helical groove in the shaft to "pump" oil back into the housing.

 

[MechatroPro]

Thanks tbuelna for your reply,
the model of the Oil flinger/labyrinth combination, should be something like this:

http://oi57.tinypic.com/2e5jc4p.jpg

but as I haven't fully understood the function of the parts yet, it might not be correct.what i'm looking for is the type of fit between the oil flinger and the shaft, and how exactly does the oil flinger diverts oil from the labyrinth?

 

[MikeHalloran]

Sometimes a flinger comprises a round, thin piece of metal sheet with a central hole pierced and extruded, and just pressed on the shaft. It typically rotates with the shaft. Any oil that migrates along the shaft to the flinger, where centrifugal force will literally fling it off the relatively sharp edge while the shaft is rotating.




Mike Halloran
Pembroke Pines, FL, USA

 

[Artisi]

Does the slinger have a groove corresponding with the internal labyrinth, from the drawing it appears so but not enough detail to be certain.
I would agree with others, it's not a lubrication slinger but an exclusion flinger limiting the amount of oil migrating between the 2 labyrinths, any oil passing the internal labyrinth probably drains back into the gearbox through the bottom of the cavity between the labyrinths.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[JJPellin]

Artisi is correct. This is a common arrangement used in gearboxes, compressors and steam turbines. The flinger is fixed to the shaft and would tend to sling the oil outward, away from the laby seal. The flinger also has an axial clearance to the face of the laby seal with a interlocking projection to make a torturous path for any oil leakage. In order for oil to reach the laby, it has to pass inward through this axial clearance, against centrifugal force and through the torturous path around the axial projection to get to the laby. You might think that this would make a very good seal. But, in fact, they often leak.

The first set of laby teeth has a drain back slot that drains oil back into the sump. This drain-back does not go all the way to the larger cavity between the two sections of the laby. That cavity is designed for the introduction of a purge gas (typically air or nitrogen). Adding a purge gas can reduce the chance of leakage. But, many of these still leak. Leakage can be affected by windage on the coupling just outside of the laby seal.

This flinger should be mounted to the shaft with sufficient interference to prevent it from spinning on the shaft or moving axially. I would recommend 0.001 inch per inch of shaft diameter. This flinger needs to be positioned to provide the required axial clearance to the laby. The correct axial clearance depends on the axial float of the gear. The drawing shows a gear with thrust faces on the inside edge of each radial bearing. This could have an axial float of as much as 0.080 inch. The flinger needs to be set such that it won't rub with the gear thrust hard toward left (as shown in the first print). Ideally, the axial clearance would be about 0.030 inch.

The second drawing seems to show a second slinger in the cavity between the two sections of the laby seal. This second slinger would be mounted with a similar interference fit and the same axial clearance as the first one.

Johnny Pellin

 

[JJPellin]

I should have said "thrust hard toward the right" in the second to the last paragraph. The axial clearance should be set with the gear in the normally running position. For this bearing arrangement, I would expect that to be the center of the axial float of the gear. But, if the gear thrusts toward the flinger, you do not want the flinger to contact the laby before the thrust bumper on the bearing contacts the thrust face.

Johnny Pellin

 

[tbuelna]

MechatroPro-

As JJPellin noted, maintaining a small axial gap between the tapered surfaces is important since the force ejecting the oil is the result of viscous shear. It is also important that the outer perimeter of the slinger ring is above the liquid oil level in the sump.

If your slinger ring is metal, then as JJPellin suggested you can locate it on the shaft using an interference fit. However, if your slinger ring is made of a non-metallic material you may need to use some other method to fix it to the shaft. While the slinger axial gap should be kept as small as practical, be sure to take into account factors such bearing axial clearance, thermal growth, and shaft deflections due to gear loads, radial loads at the output end, and shaft dynamics. Be sure to do the same for the radial shaft clearance in the labyrinth grooves.

Hope that helps.
Terry

 

[MechatroPro]

Thanks everyone, specially JJPellin, for clear explanation.
Another question I have is about the axial positioning of the shaft, is it only supported axially by the two sleeve bearing faces on an oil film? (is it sinking in oil?)

 

[MechatroPro]

The helical gear installed on the shaft induce significant amount of axial force on the shaft, if its supported only by an oil film , doesn't it squeeze the oil film out of the gap, and cause metal to metal friction?

http://oi62.tinypic.com/2uj6zcy.jpg

 

[JJPellin]

It was not clear from the drawing that this is a single helical gear pattern. Normally, I would expect a helical gear to have a more substantial thrust bearing. The arrangement in the drawing looks more like the thrust bearing I would expect for a straight gear or double helical gear. The thrust faces on the ends of the radial bearings probably have pads machined into them which act like a floating pad, hydrodynamic thrust bearing, but without the floating pads. A bearing of this type can be designed to support a substantial thrust load. The rotation of the thrust face on the gear produces an oil wedge between the rotating and stationary parts. If the speed, surface area and oil properties are correct, this will support the load required. It is no different than the hydrodynamic radial bearings which have to support the weight of the gears plus the separating forces trying drive them apart plus the forces generated by the transmission of torque.

I would prefer to see a tilt-pad, Kingsbury type thrust bearing in a single helical gearbox. It would be more forgiving and probably more reliable in the long-term.

Johnny Pellin

 

[tbuelna]

MechatroPro-

Without extensive details of your gears, bearings and operating conditions it would be impossible to say whether the contact conditions at the bearing thrust face are hydrodynamic or boundary in nature. If we assume the gear helix angle is modest (<20deg), the shaft speed is not too low (a couple hundred rpm or more), and there is a modest amount of leakage from the adjacent journal feeding oil to the thrust face, then I don't think you would have a problem. If the sketch proportions are accurate, it appears there is a generous amount of bearing area at the thrust faces.

Good luck to you.
Terry