Bearing Capacity (Spherical Roller)

[1athoti]

Hi! I have a question regarding selecting bearings based on advertised load capacity.

I have a pulley with a resultant load of 142,000 lbs (A wire rope around the pulley with 100k tension. The wire rope does not wrap around 180deg but about 90deg, so vertical up and horizontal to the bull wheels). The pulley would have a very slow speed of maximum 10 RPM. As such, both static and dynamic loads are crucial.

I was intending to use 2 pillow block bearings - though I may have to use 2 flange bearings. I asked a reputable manufacturer if the bearings are designed to withstand this direction of load since initially I was not mounting the pillow block bearings directly in line with the resultant force of the wire rope. From their catalog I have this data on a 3-15/16 spherical roller bearing.

96.5k / 71k (per bearing) = 1.35SF is not great, but it is something...



However, the response I received from them, and I quote: "Can't even come close to handling the 72K with a 3-15/16 bearing. In fact even a 6-15/16 could not handle it. The "load rating" shown on the web site is a reference value only. In reality, only 25% of this value can be loaded to the bearing. This is a industry standard practice." (my bold and underline)
Is this truly so? I am no expert on bearings, do companies really advertise 400% more loading than they should??

Thank you for your input!

 

[dgeesaman]

Static load rating is for non-moving cases only such as emergency / upset conditions. Rolling a bearing anywhere near the static load rating will cause failures in short order.

Service factor has no applicability to rolling bearings, at least since bearing life calculations were developed in 1947. What is your desired L10 life? And is it acceptable to have 10% of the bearings failing before that time?

Bearing L10 life relates life to load on either the ^3 or ^(10/3) power. In my industry, our equipment sees mostly continuous use and customers request anywhere from 40k to 300k hours of L10 life.** A working load of 25% of the dynamic rating gives 100k hours of L10 and, lacking any other context, Rexnord is giving you a good starting point.

** L10 life is the original and simplest bearing life estimation method, based on bearings tested at 100rpm in one specific set of lubricative and environmental conditions. L10a is a more refined method and I encourage you to use that because speeds < 10rpm will less ideal lubrication than 100rpm and will require a bigger bearing size to give acceptable life. So the more accurate L10a calculation is likely to be fewer hours than the 100k hours given by L10.

Roller bearings take load in two directions: axial and radial. It sounds like your pulley takes only radial load. The angular direction of the radial load shouldn't matter much as long as it's mounted sufficiently well to support the loads. Spherical roller bearings have no rigidity against out-of-plane bending - so if you're intending for your wire ropes to enter/exit the pulley not in a single plane, then you'll need to perform the statics calculations to resolve the load into an axial and radial load on each roller bearing.

Which brings me to another subtlety: spherical roller bearings can take heavy axial loads, sort of. The risk is that when they reach their failure point in axial loading, they tend to fail very suddenly and either seize or blow apart. That is often an unacceptable scenario, which is why I don't always reach for a spherical roller even if it looks to have good L10 performance on paper.

 

[BEMPE16524]

that is quite big load (142,000lbs = 64 Tonne). I'm not going to use 1.35 safety factor for that.
I would rather oversize my design to at least 1.5-2.
you should consider to ensure reliability and durability under potential peak loads, alignment variations, or dynamic effects.

 

[1athoti]

Thank you @dgeesaman and @BEMPE16524 for the input!

These would be bearings on the fairlead wheel on an underground cable puller. Yes, 1.35 SF is much too low, totally agree. The wheel has slow RPMs and could be fully loaded at stand still. That's why I was mentioning Static Load capacity also. The Static Load capacity of the bearing is actually higher giving a 1.93 SF.

The machine would only be used 500 - 800 h/year so fairly light usage. But to have a failure at the wrong time can be very costly.

I looked into it a little more and I'm afraid for the space I have and high loading I will have to look towards a solid bushing, bronze or other alloy. A failed roller bearing would mean a standstill for the job until repaired. A 'failed' bronze bushing (too worn out) can still finish the job and be replaced later in shop etc.

 

[dgeesaman]

Spherical rollers tend to fail suddenly only in high axial load conditions. In radial load conditions they tend to truck along.

Will they receive regular inspection and relubrication?

 

[BEMPE16524]

Thank you @dgeesaman and @BEMPE16524 for the input!

These would be bearings on the fairlead wheel on an underground cable puller. Yes, 1.35 SF is much too low, totally agree. The wheel has slow RPMs and could be fully loaded at stand still. That's why I was mentioning Static Load capacity also. The Static Load capacity of the bearing is actually higher giving a 1.93 SF.

The machine would only be used 500 - 800 h/year so fairly light usage. But to have a failure at the wrong time can be very costly.

I looked into it a little more and I'm afraid for the space I have and high loading I will have to look towards a solid bushing, bronze or other alloy. A failed roller bearing would mean a standstill for the job until repaired. A 'failed' bronze bushing (too worn out) can still finish the job and be replaced later in shop etc.

yup, a free maintenance bearing like Lub-met can be considered. also, not sure how big and thick is your pulley. just make sure it will not wobble on your shaft. considering 800hr/yr, which is around 2hr 12min per day, or around 5.5min every hour, that is pretty less usage.

 

[dvd]

Attached is some information from SKF on bearing housing loading. Housed bearing manufacturers have this type of literature available,