There are at least two major drawbacks to hollow shaft reducers:

1. CORROSION: oxidation of shafts, fuses the two metal shaft surfaces together, makes it difficult to exchange the reducer assembly after a few or more years have past, unless the environment is dry. Grease packing is not encouraged, since a clamping pressure is normally used to transmit torque.

2. RESTRICTED SPACE: Reducer must be removed over the extended shaft length engaging the width of the reducer plus pressure connection. Not good for underground mining and other close spaces. On a minor point, an extended pulley shaft increases shaft run-out tolerance allowance errors, plus it can create shipping and handling difficulties.

Much above 200 kW, major mining companies discourage hollow shaft usage due the corrosion issue. ■

Some 20 years I was involved with a prototype machine that used two slow speed shaft mounted hydraulic motors that used 'shrink disc' fixings. Later we needed to reuse the motors on another job. But first the bolts on the shrink disc were seized and we had to cut through them, only to find that the motor hub was seized onto shaft. We ended up cutting through the shafts and then having to machine them out to reuse the motors. NEVER AGAIN!

Now I only use shaft mounts with keyed connections and plenty of grease! ■

Dear Designer,

I see we have different experiences. The hollow shaft can also use either a shrink disk, external to the shaft end, extended beyond the hollow-shaft reducer, or an internal RFN type. Keyed units still may have corrosion on their length wise surfaces between shaft and hub.

Even lots of grease does not fully or completely protect against corrosion welding of the shaft and hollow bore.

We prefer to use hydraulic apply and release internal sleeves on the two halves: one on the reducer low speed shaft and one that connects to the pulley shaft.

It is impractical to use keyways on large shafts greater than 400mm diameters. You give too much away in shaft strength. Even smaller shafts (<300mm diameter) do benefit from non-keyways connectioning pulley hub to shaft, and pulley low speed couplings between pulley and reducer. ■

A clarification:

Hollow shaft reducers have small but perceptable movement that tend to burnish the shaft to hollow bore contact surface which canwipe clean the greased areas. Corrosion then attacks the non-greased surfaces creating a welded bond. ■

Sirs,

As rightly pointed out, holllow shaft gear boxes can pose a maintenance problem when you need to remove the entire gear box for overhauling or so.

I think the real use of hollow shaft gearbox is when the movement of driven part with respect to drive base is considerable. And in those cases only one should opt for hollow shaft gear box. ■

Luckily my more modest shaft sizes (compared with 400 mm) can accept a keyed connection without problem, and so far (touch wood) our greasing techniques have not resulted in a complete seizure of the gearbox on a shaft. ■

Anyone who applies plain lubricating grease to a hollow shaft mounting is asking for trouble. Most grease is slightly acidic, and when water inevitably penetrates the interface between shaft and bore it will rust real good.

We always use an anti seize compound specifially made for this purpose and the results are usually good up to about 6" diameter. After that you should be looking at shrink disk or drive disk connections just because of the weight involved.

The main advantage of shaft mount is no coupling alignment and no misalignment after the floor steel flexes under load and your previously aligned coupling is suddenly not in alignment any more. ■

Dave,

FYI-

The Cardan shaft that Gary installed at KPC has been modified to lock the separated motor base and reducer base together via a third swung platform. This makes for a single base. So, why the added complication, instead of building one new base? Beats me.

They still kept the Cardan coupling with its extension.

This configuration was presented at the Salt Lake City 2008 annual SME convention. Edmond O;Donovan, Prof. Craig Wheeler, Prof Gabriel Lodewijks, Mr. Alspaugh, and many others were in attendance. ■

A genuine cardan shaft might give simpler alignment but it also gives output velocity fluctuations.

Were these superimposed fluctuations considered in the shaft design; as far as anyone knows?

I must admit I've only ever put smaller shaft mounted speed reducers on indoor jobs. But if they've rusted up in the passing years just let me say "I'm a little bit sorry" and " Put yer backs into it ya lazy gits!" ■

Hi all..

I have had good success using extended gear couplings instead of cardan shafts.

It was an idea that Johan Rall of Hansen gave me ages ago for the 3.4MW single drive for the Majuba incline conveyor.

He said all you need to do is put an extension spool piece in the middle of the gear coulpling. Sort of like a double coupling in series.

This takes considerable misalignment and acts like a cardan.

I used recently it on the Sishen Iron Ore Mine primary incline conveyor drives.

It works very well.

Other than that, having battled with a D10 dozer trying to pull a hollow shaft 500kW shaft mount off an in-pit conveyor drive drive, I agree with the above condemnation of hollow shafts for any meaningful drive size.

Regards

LSL Tekpro ■

I have used many hollow shaft, shaft mounted reducers, both right angle with swing base, or scoop or flanged motor mounts, and parallel shaft, direct and V-belt driven. The hollow shafts have been keyed or keyless, with tapered bushings or shrink discs.

I am not aware of the many problems in removal after the fact so my comments will not speak to this. I do love the simplicity of any shaft mounted drive, especially the right angle units largely because of the simplicity in installation and the alignment free nature of the system. As a young structural engineer I remember the repeated deflection analysis and redesign of the supporting structure under a LS coupled parallel shaft drive, at a CBU collecting conveyor. I was never able to meet the published deflection tolerance for the LS coupling but my supervisor said it was fine and it was built. How much simpler is a shaft mounted drive, connected only to an anti-rotational torgue link (strut)?

The simplicity and merit of shaft mounted drive systems was discovered long ago and used to a great advantage at very large drives(to 5000 kW) of the brown coal fields of Germany.

My two cents.

Joe Dos Santos ■

I have been working in the salt industry for 30 years the last 12 in the salt mines and we have no problems with hollow bore shaft while using a shrink disc.

We use shrink dics for all reducers over 150 HP and tapered bore hollow shaft for 150 HP and under.

Although it is not recommended to use any type of lubricant on a hollow bore shaft mount reducer we do use a product called "Never Sieze" on the shaft which will allow removal of the reducer from the shaft. Without it they will sieze permanently to the shaft. ■

we manufacture hollow shaft mount speed reducers

being aware of fretting corrion problems in shaft mounts , several years back we manufactured the shaft mounts with stainless steel hubs with a keyway.

The market did not accept them well due to the higher costs involved

We started manufacture of the keyless hub shaft mounts with taper clamp arrangements and the taper clamos are now made in stainless steel 420 grade , which has the required strength as well as good fretting corrosion resistance

the market has accepted these very well

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