In this article we will discuss about:- 1. Requirements of Gear Boxes for Machine Tools 2. Methods for Changing Speeds in Gear Boxes 3. Feed Gear Boxes 4. Infinitely Variable Gears.
Requirements of Gear Boxes for Machine Tools:
A gear box for machine tool should possess the following requirements:
(i) Provide adequate spindle speeds.
(ii) Transmit desired power.
ADVERTISEMENTS:
(iii) Provide smooth silent operation of the transmission and accurate rotation of the spindle without vibrations.
(iv) Simple construction in respect of total number of shafts, gears, clutches, bearing and control system components.
(v) Ease of carrying out preventive maintenance and to make the adjustments in bearing clutches etc. by taking care of easy access.
The gear box may be built either into the spindle head (or head stock), or be designed as a separate unit.
ADVERTISEMENTS:
Gear boxes designed as a separate unit integral with spindle head provide a more compact spindle drive, are easy to assemble.
However, in these, vibrations from the gear box may be transmitted to the spindle gear boxes and also heat produced in the gear box may heat the spindle head.
In the case of gear boxes with a divided drive, the gear box and spindle head (head stock) are designed as separate units and the gear box is linked to the spindle head through some type of transmission such as belt transmission. In these gear boxes the heat produced in the gear box by friction losses and vibrations are not transmitted to the spindle head.
Methods for Changing Speeds in Gear Boxes:
The speed of gear box can be charged by engaging the various transmissions in gear boxes by change (slip) gears, or sliding gears, or sliding key mechanism or using friction clutches. Change gears are used when the spindle drive is to be changed infrequently.
ADVERTISEMENTS:
Lot of time is lost in changing speeds. Gear boxes with sliding gears (Fig. 11.50) are commonly used in general purpose machine tools because of their simplicity and reliability. Gear boxes with jaw clutches are commonly used in heavy machine tools. Friction clutch for changing speed is used mainly in the group transmissions of small and medium size turret lathes.
Sliding gears type gear boxes (Fig. 11.51) can transmit heavy torque and can run at high speeds. These gear boxes have sliding cluster gears and the gears not participating in the transmission of power to the spindle in a given engagement are not in mesh.
Feed Gear Boxes:
ADVERTISEMENTS:
Feed gear box is located in front to change the feed rate as desired for given application. The drive of the feed mechanism may be powered by a separate electric motor or from the head stock spindle through a gear, chain or belt transmission.
Various drives for feed gear boxes are discussed below:
i. Norton Gear Box:
It is commonly found in engine lathes. The different speeds of driven shaft are obtained by engaging the tumbler gear with each gear cone fitted on the driving shaft. The sliding gear is keyed to the driven shaft and meshes with the tumbler gear which is held in a bracket pivoted on the driven shaft.
ADVERTISEMENTS:
Fig. 11.52 shows a better version of Norton gear in which one gear is slide on splined input shaft to obtain different speeds by engaging desired gear on intermediate shaft.
ii. Feed Gear Box of Meander Type:
It is a three shaft mechanism made up of a series of identical double cluster gear and a sliding gear with a tumbler gear. In this gear box all the cluster gears rotate continuously in mesh including cluster gears which do not participate in a particular engagement.
iii. Clutched System:
A clutch can be used to transfer power from the driving shaft to feed screw shaft. A clutch can slide on splined shaft.
When the clutch is connected at left the speed (N1) of feed screw shaft is given by:
When the clutch is connected to right gear the speed (N2) of feed screw shaft is given by:
iv. Ruppert Drive:
It is a clutched drive using two clutches and six gears 1, 2, 3, 4, 5 and 6 to provide 4 speeds to feed screw shaft.
When clutch 1 is towards left and clutch 2 is also towards left, the output shaft will have one speed. When clutch 1 is towards right and clutch 2 is towards left the feed screw shaft will have second speed.
When the clutch 1 is towards right and 2 is also towards right the feed screw shaft will have third speed and when clutch 1 is towards right and clutch 2 is towards right the feed screw shaft will have fourth speed. A typical ray diagram for this drive is shown in Fig. 11.55 and actual cross sectional diagram is shown in Fig. 11.56.
v. Pre-Optive Gear Box:
Such gear boxes are quite commonly used in capstan and turret lathes. In a pre-optive gear box the operator preselects the speed on a dial and then pulls a lever to obtain it. A clutch is operated to get the desired speed without stopping the machine.
Thus where frequent speed changes are necessary this type of gear box is quite helpful. The arrangement of an eight speed gear box is shown in Fig. 11.57 (a) and Fig. 11.57 (b) shows ray diagram for the same.
Infinitely Variable Gears:
The infinitely variable drives find many applications in modern metal cutting machine tools.
The advantages of these drives are:
(i) Optimum cutting speeds and feeds with higher accuracy can be set.
(ii) The speeds can be changed rapidly without stopping the machine.
(iii) A more uniform quality of surface finish is attained.
(iv) Operation is quiet.