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Load Carrying Capacity of Revolving Center's

A recent Query in the rec.craft.woodturning News Group, asked, what were the Morse Taper Live Centre Limitations and the particular query was related to a 1500lb (680+kg) Load requirement.

This was an interesting question and from memory no one came up with a definitive answer only historical experiences and hear say, many of the replies related to the disasters experienced or heard of  relating to large "Chunks" of Timber destroying this Back End Support.

Many of the replies related to the disasters experienced or heard of  relating to large "Chunks" of Timber destroying this Back End Support.

Investigations revealed a number of factors and some hard figures of possible Load Bearing Capacities relative to the Size of Morse Taper employed.

Firstly although the actual Morse Taper has a relevance on the final Carrying Capacity this is as much due to the Physical size of Bearing that could be applied as it is to the size of the Taper, however the final capacity is determined by not only the size of Bearing but by the Type of Bearing(s) used and even the Construction of the Revolving Centre itself.

Secondly looking through all the major UK and USA Suppliers Catalogues there is no information given for the Capacity of the Revolving Centres on offer and by far the most common sizes being #1 & #2 although many of the Larger Lathes do in fact have a #3 Morse Taper in the Tailstock Barrel.

I came across some actual Carrying Capacity Data in an Engineering Suppliers Catalogue and having talked with them I now have a better understanding of what dictates the Final Load Capabilities of any Revolving Centre.

The Factors are:-

By bearing type you are looking primarily at the basic construction of the Bearing which can be either:-

These types of Bearing offer a varying degree of Support for Radial Loads with some forms also giving a degree of Thrust Support although it is more common to fit a separate Thrust Bearing.

The Ball Bearing is the least effective Bearing Type (see tables below) and comes in two basic forms, Light and Medium Duty with various options on Groove Depth in which the Balls Locate.

Next we have the Roller Bearing that uses Cylindrical Rollers in place of the Balls in the Bearing above this allows for a larger contact area and therefore an increased Loading.

Along side these are the Taper Roller Bearings which offer all the advantages of the Roller Bearing plus a high degree of Thrust Support but not necessarily with any additional Load Carrying capabilities particularly on larger sizes.

Finally we have the Needle Roller Bearing which has Smaller Diameter Bearings but Longer and this is generally considered to be the ideal Bearing for fluctuating Loads and Precision Applications.

An important aspect is the Construction of the Revolving Centre and two formats are in general use, type 1 is where the Body of the Centre is fixed and the Cone Part of the Centre rotates, and type 2 where the Spigot of the Morse Taper supports the Bearing and the Cone is part of the Bearing Housing which rotates.

Of the four Revolving Centres I own three are of the type 2 and only the one is of type 1, this happens to be my favourite Centre and until I started this research I was not aware of why this should be, now I appreciate that type 1 Centres are the Higher Quality, Higher Specification and of course Higher Priced type.

It appears that most Engineering Centres are of type 1 design and for really High Load Designs these can also incorporate an additional Bearing at the far end of the Morse Taper to give added stability.

The Carrying Capacities given below in Table 1 are those for the various types of bearing to show the difference between them, this is based on a Bearing I took from a #2MT Revolving Centre that I was able to disassemble. However I have four #2MT Revolving Centres in total and it is obvious they all have a wide variation in the Size of Bearing(s) fitted so the data in Table 1 cannot be taken as applicable for all #2MT Size of Revolving Centre.

The actual Bearing in question was  35mm od with a 16mm bore and appears to be of the  Roller type Bearing - Light, and therefore according to the table I should be able to operate this up to a maximum Loading of 400kg (@100rpm), and this appears to be adequate for the range of Turning I do. Incidentally no Thrust Bearing was fitted to this particular Centre.

Bearing Type

Carrying Capacity @ 100rpm

Carrying Capacity @ 1500rpm

Ball - Light 510lb / 230kg 210lb / 95kg
Ball - Medium 820lb / 370kg 330lb / 150kg
Roller - Light 890lb / 400kg 450lb / 205kg
Roller - Medium 1450lb / 660kg 730lb / 330kg
Taper Roller -Light 1250lb / 570kg 640lb / 290kg
Taper Roller -Medium 1800lb / 820kg 920lb / 420kg

Table 1

I have no figures for the Needle Roller version of the above but would expect it to surpass any of the ratings for the other types of Bearing.

The Standard method of rating a Bearing as detailed above is based on a 500 hour Life only. There are many other factors that are put into the "Selection" Equation (Factors for, Load, Speed, Life and Application) for Bearing Selection and one assumes that Designers of Revolving Centres take this on board.

To actually determine which type and which grade of Bearing you have within your Revolving Centre may be difficult, if not impossible, to establish, so an educated guess will be required if you ever need to know whether the Lump of Wood you have just mounted on the Lathe is going to Destroy your Revolving Centre or Not?

Incidentally the rating for a similar size of Thrust Bearing is approximately 900lb (410kg) at 100rpm and therefore does not appear to be a determining factor as I'm sure the amount of Thrust Load experienced is far less than the Radial Load.

The next table gives the Ratings stated in the Catalogue I have for a High Precision, Heavy Duty Centre with Roller Bearings and a Ball Thrust Bearing with a Seal to prevent any ingress of Dust and loss of Lubricant.

Shank Size 50rpm 150rpm 400rpm 1000rpm
#1MT *      
#2MT 340kg 240kg 180kg 130kg
#3MT 400kg 340kg 300kg 260kg
#4MT 1300kg 900kg 650kg 450kg
#5MT 2000kg 1300kg 920kg 500kg
#6MT 4000kg 3600kg 2600kg 1900kg

Table 2

* #1MT Size is rated as being suitable for Light Work Only and does not include a Thrust Bearing as part of the assembly.

To convert kg to lb multiply above figures by 2.2 i.e. 340kg = 748lb.

Although not clarified any where in the information I have, it is assumed (by me) that the Loadings are based on a Uniform Centrifugal  Load on the Bearing(s) this may not be the situation with an odd shaped piece of Timber and therefore the figures above should be considered as the absolute maximum.

You will note that the Loadings given for the Basic Bearing in Table 1 are higher than those for the Revolving Centre in Table 2 and I have no explanation for this, but it is possible that the ratings in Table 2 are based on a longer Bearing Run Life, as most Revolving Centres carry a 1 year Warranty, which equates to well in excess of 2000 hours of possible use [approx 6 hrs. a day] in this period (Average Industrial use) and therefore a Correction Factor will be applied to the Bearing Capacity which are rated at 500 hrs. of life.

If you are concerned about the Revolving Centre you use then ask yourself what is the maximum load you are likely to use, bearing (no pun intended) in mind it would take at least three people to put a 230kg (worse case in table 1) Lump of Timber on the Lathe (two to lift it and one to bring up the Tailstock).

If you are still concerned then try your local Engineering Tools Supplier who may have a more detailed Specification for the range of Revolving Centres he can supply. Incidentally in the UK I could purchase the #2MT Centre in table 2 for 50 and a #2MT of unknown Specification for 30 to 35, think I know where I will get my next Revolving Centre from if I ever need to change.



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