This section is from the book "Lathe Design, Construction And Operation, With Practical Examples Of The Lathe Work", by Oscar E. Perrigo. Also available from Amazon: Lathe Design: Construction And Operation.
The manufacturer of the lathe says: "Our aim in its design has been to provide this power in such a manner that all the functions of the regular type would be retained, but the head would have wearing qualities, in addition, proportionate to the increased service expected of it. To this end we believe the observance of the following conditions to be of the highest importance: First, the spindle bearings, upon which the accuracy of the lathe is dependent, should not be subjected to the change of alignment by carrying the pull of the belt. Second, more force at the cutting tool should be secured by the use of wider belts, instead of through higher gear ratios. Third, the possibility of running the lathe 'out of gear' should be provided for in cases where finishing cuts are desired. Fourth, speed changes should be secured without the necessity of shifting belts. Fifth, the lubrication of the bearings should be automatic and positive."
Doubtless every thoughtful mechanic will readily assent to these propositions as being self-evident.
Figure 273 shows the head-stock in place upon the bed and with the main spindle and the gear covers removed in order to show the construction of the driving mechanism. Power is applied through a wide-faced pulley of large diameter which is keyed to a sleeve revolving in the two central bearings of the head-stock. At one end of this sleeve is a jaw clutch, and at the opposite end two gears of different diameters. The main spindle passes through this sleeve without coming in contact with it, having about an eighth of an inch clearance, and revolves in the two outer bearings, that is, the extreme front and the extreme rear bearing. It is connected to the driving sleeve for direct belt speeds by the clutch, and for the back gear speeds through either back gear, according to the speed desired. A lever, convenient for the operator, engages or disengages the clutch.
Fig. 273. - Head-Stock for Lodge & Shipley Patent Head Lathe.
As there is no contact between the driving sleeve and the spindle except through the clutch, the pull of the belt is all carried by the two central bearings. Sufficient clearance is provided in the clutch to prevent any of the belt strain being communicated through it to the spindle. The spindle bearings are thus relieved of all wear due to belt pull and their life greatly prolonged. By actual experiment with a 20-inch lathe it has been shown that the pressure exerted by a belt on spindle bearings was 17.6 pounds per square inch of bearing surface, while the total pressure exerted by the belt upon a spindle between bearings which effect the alignment of the spindle was 393 pounds. In the lathe under consideration this was entirely eliminated.
In the ordinary type of engine lathe the narrowness of the driving belt compels the use of the back gears for all cuts but the lightest ones, and on small diameters. To provide sufficient force at the tool for heavy cuts, this back gear ratio must necessarily be a high one, and, as the speed at which the cut is taken is reduced in the same ratio as force is gained, it is apparent that a heavy chip cannot be removed at a high speed unless the speed of the cone pulley is increased to an enormous rate. When this is done, the fact that it revolves directly on the spindle, where it is impracticable to maintain an adequate supply of oil, soon causes excessive friction and is liable to stick the cone pulley.
In the lathe we are considering the great width of belt used delivers sufficient force at the cutting-tool for heavy cuts through a comparatively low back gear ratio, in consequence of which the spindle speeds may be proportionately higher. An additional set of back gears of very low ratio is provided for cuts which are slightly beyond the capacity of the open belt, but which do not require the full force afforded by the high ratio. Thus it will be seen that high speeds can be secured through the back gears without the necessity of revolving the driving pulley at the enormous rate required of a cone pulley to perform the same work. In addition the construction of its bearings is such as to permit of perfect lubrication, which has received a great deal of attention, and the manufacturers claim that the spindle will run a month with one oiling. Deep oil wells, holding about a pint each, are formed in the casting under the centers of the bearings of the spindle and driver sleeve, and are connected with gage glasses at the front of the head-stock for the purpose of showing the height of the oil. The oil wells are filled through these gage glasses, which allows any sediment or dirt which the oil may contain to settle to the bottom and not be deposited on the revolving journals where damage would be liable from cutting. At the center of each journal is attached a brass ring with four projections, on the principle of the bucket pump. As the journal revolves these buckets dip into the oil in the well, and, passing over the center of the bearing, pour the oil over the journal. Suitable ducts distribute the oil lengthwise of the bearing and return it to the well to be used again and again. This method provides a certain system of lubrication without regard to the speed of the revolving spindle.
The back gearing is designed with ratios to give a uniform progression of speed from the slowest to the fastest. The two back gears are connected to the back gear shaft by spline and key, and are easily moved lengthwise to engage with their respective gears on the driving sleeve. The back gear shaft and pinion are made of forged steel, thus insuring the requisite strength and wearing qualities. The journals for the shaft are placed at either end, where they revolve in bushings provided with oil reservoirs and the same system of oiling as that for the spindle and driving sleeve.