Page 70 - Robotics Designing the Mechanisms for Automated Machinery

2.4 The Kinematic Layout 59

This ratio can be apportioned between the belt drive and reducer in, say, the fol-
lowing way:

i = z r i 2 =1.25-24 = 30, [2.5]
where the ratio of the belt drive z\ = 1.25 and that of the worm reducer i 2 = 24.
The gears 8 obviously deliver a 1:1 ratio, the wire-pulling rollers 6 and 7 being of
identical diameter. Another important point we have to mention here is that the kine-
matic solution discussed above is not flexible. For instance, to add more coils to the
end product we must increase the diameter of wire-feeding rollers 6 and 7, causing
more wire to be introduced into the machine and producing more coils per spring. To
change the pitches along the spring, cam 9 must be replaced by a corresponding cam.
Note, however, that substitution of these elements of the kinematics entails relocat-
ing corresponding shafts and their bearings, in addition to relocating the guides of the
wire 4 (see Figure 2.4). Briefly stated, the proposed concept restricts the flexibility of
the machine. The only parameters which are easy to modify are the diameter and con-
stant pitch of the spring, thanks to the design of the supports (5,6, and 7 in Figure 2.4).
This difficulty can be overcome by adopting a different concept of the kinematic
layout (assuming a higher degree of flexibility is needed). One possible solution is rep-
resented in Figure 2.18. Here the systems of the automatic machine are kept separate,
the feeding mechanism having its own drive while the cutter and bending tools are
moved independently. In more detail it can be explained in the following way. The
motor 1 drives the feeding rollers 3 through a worm-gear reducer 2: as in the previous
case the rollers are engaged by a pair of cylindrical gear wheels 4. An electromagnet 5
drives the cutter 7 along guides 8 with the help of a lever 6: the return of the cutter to
its initial position is accomplished by a spring 9. The tools 10 shaping the spring (one
or several) are fastened in corresponding guides 11. These guides can be moved along
axis X (the tools are fixed to the guides by means of bolts 12). An independent motor
13 is used to carry out this movement. This motor drives link 14 which consists of a
nut restricted in its axial movement and therefore able to realize pure rotation only.
The thread of this nut is engaged with a lead screw 15. The latter, in turn, is restricted
in its angular (rotational) motion by means of a key 16. Thus this screw realizes a pure
axial motion, driving also the tool 10. The designer decides how many tools are to be
driven independently. Analyzing this new kinematic layout and comparing it with the
previous one, we arrive at a significant conclusion. The second layout permits easy
modification of the duration of action of the motors, thereby delivering any (reason-
able) length of spring, coil number, or pitch. For this purpose the control of the two
motors and electromagnet must be correspondingly tuned. Obviously, instead of elec-
tric motors, hydraulic or pneumatic drives could be installed; the control unit would
then have to be designed to fit the nature of these drives.
Here we must return to Chapter 1, Section 1.2, and analyze the examples given
there in terms of the diagram given in Figure 1.5. The purely mechanical kinematic
layout (Figure 2.17) clearly belongs to level 5. Indeed, the energy source is a motor and
control is carried out in series by the kinematics (transmission, cams, and levers) of
the system. Considering the case given in Figure 2.18 we see that the machine con-
sists of at least three systems of level 6 (Figure 1.5). Two motors (1 and 13) and an elec-
tromagnet 5 impart the driving energy. In addition the motor 1 drives the program
carrier, which consists of a conical speed variator comprising a cone 17 and friction-