Slewing Bearing Design Question 1

[scrappy79]

I am sizing a slewing bearing. I am considering static and dynamic moment reactions (tilting) into the slewing bearing. When looking at moment charts they are always static moment charts. What is the typical practice for determining if the dynamic loads will not exceed the static loads shown on the static load chart. I'm pretty sure all static load charts incorporate some type of safety factor but it's not always notated on the chart. Can you use the safety factors on the static load chart to incorporate your dynamic loads? In this case the dynamic loads would only come into play if there was a maximum static load and then some dynamic load was introduced to that maximum static load.

 

[dhengr]

Scrappy:
Since you have given so little in the way of real info. about your actual problem, sketches, dimensions, loads, moments, sizes, etc. it is pretty tough to comment in any detail. We don’t know if you are dealing with a 1 ton machine or a 1000 ton machine. There are some specs., industry standards, or manufacturer self imposed criteria for some of this kind of design. But, I can’t point you at one specific one off the top of my head. I would take your questions to several of your suppliers, they know their products better than anyone else does. Talk to their engineering people not a sales person. Then you have to be smart enough to know when they are trying to blow smoke up you leg.

 

[tbuelna]

scrappy79,

I think you might be confusing your terms. With regards to rolling element bearings the terms "static capacity" and "dynamic capacity" are used to differentiate whether the bearing itself is rotating. When rotating above some marginal speed, the rolling element/raceway contact becomes hydrodynamic, which improves its load carrying capacity. That's why published rolling element bearing dynamic capacity tends to be greater than static capacity.

Slew ring bearings present a unique analysis situation. They have very narrow axial length, but are designed to carry combinations of axial load and out-of plane moments. They are usually operated at slow rotational speeds, or with a reversing narrow angle oscillatory motion. And most importantly their designs do not usually permit installed preload. All of these characteristics require consideration when calculating dynamic bearing life.

The bearing cannot tell whether the applied forces/moments are dynamic or static in nature. With bearing fatigue life what is of concern are the number/magnitude of load cycles each sector of bearing race surface is subject to, as well as the contact conditions. As noted above, slew ring bearings do not usually have installed preload. When large out-of-plane moments are applied the number of rolling elements on opposing race sectors that actually carry load can become quite small (a ball/roller can only transfer load in compression, right?). This effect can be offset somewhat by the combination of axial (thrust) load on one side of the bearing. So when calculating bearing fatigue life, you need to determine a composite load/lifecycle value based on the various load/speeds the bearing components experience over their lifespan.

Lastly, the worst load condition a slew ring can experience is a large, repetitive out-of-plane moment while not rotating. This type of condition will produce tiny grooves in the race surface and flats on the roller/balls. I've attached a page out of a slew ring bearing catalog that gives dynamic L10 life in terms of bearing revolutions. Just remember that these published numbers are usually quite conservative. And these values will usually become substantially smaller when all of the recommended adjustment factors are applied. As a rule of thumb, the published static capacity of a bearing should never be exceeded under any conditions, since this would result in brinelling of the race surface.

Hope that helps.
Terry

 

[dinjin]

I would suggest that you contact several of the slewing ring manufacturers and depending on the application, the static capacity will be much higher than the dynamic for the simple reason that slewing bearings often operate beyond the calculated first signs of brinelling. The service life of many of these are 5 times the dynamic life calculations.