A live center (or revolving center) is a free turning lathe center used at the non-driven or tailstock end of the lathe.
Because it turns on its own bearing, it allows higher turning speeds without the need for separate lubrication, and also greater clamping pressures.
In this article we’ll be examining the weight capacities of live / revolving centers.
Factors Determining Weight Capacities
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 Center.
Secondly looking through all the major Suppliers Catalogs there is no information given for the Capacity of the Revolving Centers 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 Catalog and having talked with them I now have a better understanding of what dictates the Final Load Capabilities of any Live/Revolving Center.
The Factors are:-
- Bearing Type
- Bearing Size
- Bearing Grade
By bearing type you are looking primarily at the basic construction of the Bearing which can be either:-
- Ball Bearing
- Roller Bearing
- Taper Roller Bearing
- Needle Roller Bearing
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.
Construction of the Revolving Center
An important aspect is the Construction of the Revolving Center and two formats are in general use, type 1 is where the Body of the Center is fixed and the Cone Part of the Center 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 Live/Revolving Centers I own three are of the type 2 and only the one is of type 1, this happens to be my favorite Center and, until I started this research, I was not aware of why this should be.
Now I appreciate that type 1 Center are the Higher Quality, Higher Specification and of course Higher Priced type.
It appears that most Engineering Centers 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.
Capacities for Various Types of Bearings
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 Center that I was able to disassemble.
However I have four #2MT Revolving Centers 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 Center.
The actual Bearing in question was 35mm od with a 16mm bore and appears to be of the Roller type Bearing – Light
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 Center.
|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|
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 Centers take this on board.
To actually determine which type and which grade of Bearing you have within your Revolving Center 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 Center 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.
High Precision, Heavy Duty Center Capacities
The next table gives the Ratings stated in the Catalog I have for a High Precision, Heavy Duty Center with Roller Bearings and a Ball Thrust Bearing with a Seal to prevent any ingress of Dust and loss of Lubricant.
* #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 Loading amounts 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 Loading amounts given for the Basic Bearing in Table 1 are higher than those for the Revolving Center in Table 2.
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 Centers carry a 1 year Warranty, which equates to well in excess of 2000 hours of possible use in this period (Average Industrial use) and therefore a Correction Factor will be applied to the Bearing Capacity
If you are concerned about the Live/Revolving Center 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).