IBM 727 Manual Of Instruction Download

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source and the cell. However, if the tape breaks, the light source strikes the cell
and stops the machine.

4. 5 REEL DRIVE CLUTCHES

Each tape reel is mounted on a hub protruding from the upper front plate of the

machine.  This  hub  contains  an  expansion  rim   that  grips  the  reel  tightly when the
locking  screw  in  the  center  of  the  hub  is  tightened.  The  hub  is  mounted  on  a  shaft
extending through the  upper  front  plate  and  to  the  rear.  This  shaft  is  driven by  two
continuously  running three-phase  motors  through  two  magnetic  powder  type  clutches;
one  for  forward  motion  and  one  for  reverse  motion.  A  third  clutch  is  used  as  a

brake.

Figure 10 shows a single clutch unit. The innermost part is the rotor that is

keyed to the shaft. Surrounding this part is the clutch housing with pulley grooves

cut  in  its  circum ference  to  make  it  free  to  turn  on  the  shaft.  Within the  clutch  hous
ing  is  embedded  a  coil,  the  connections  for  which  are  brought  out to  slip  rings  on  one
end  of  the  housing.  In the  space  between  the  rotor  and the  housing  is  a  mixture  of

magnetic iron powder and graphite.

When current flows through the coil in the housing, flux is produced. It solidifies

the  iron  and  graphite  mixture  and  causes  the  rotor  and  housing to  be  essentially  locked
together.  Therefore,  if  the  housing  is  made  to  turn  continuously through  pulley  action
from   the  m otors,  the  rotor  does  not  move  with  it  unless  current  is  flowing through  the

coil. As soon as current is caused to flow through the coil, the rotor begins to move

with the housing and to turn the hub and reel at the front of the machine.

Because of the inductance of the coil, current in it builds up gradually and, b e

cause  torque  transmitted  is  proportional  to  current  (and flux),  a  quite  smooth  a ccel
eration  is  produced.  This  smoothness  is  essential  because  shock  in  the  reel  causes
slippage  of  the  tape  on the  reel,  and  eventual  breakage.  The  magnetic  powder  clutch

was  selected  here  for  its  ability  to  produce  quite  smooth  accelerations  and  large
torques  with  relatively  small  control  currents.  The  iron-graphite  mixture  does  not
wear  the  clutch parts  but  only  tends  to  polish  them.  To  hold  the  iron-graphite  mixture

in  the  vicinity  of  the  flux  gap,  a  series  of  labyrinth  type  ridges  were  designed  into  the
clutch parts.  These  ridges  assist  centrifugal  force  to  return  the  powder  back  to  the
magnetic  gap.

All three clutches are mounted on the same shaft with all rotors keyed to it. The

front clutch has a stationary housing and thus serves as a brake when necessary. The
middle clutch housing is driven in a clockwise direction and serves as a forward drive.

The rear clutch housing is driven in a counterclockwise direction and serves as a

reverse drive.

Control of the clutch energization is discussed under section 7. 0.

Each brake may be released when the machine is unloaded by depressing the r e

lease  button  located  below  and  to  the  left  of  the  file  reel.  The  left button  causes  d e-
energization  of the  left brake  clutch  and  the  right  button  causes  de-energization  of  the
right  brake  clutch.