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Under construction
Brakes and Wheels
Contributions by Richard
E Fulwiler
Original brake unit designs on the A-26 were of the
expanding bladder type. These were prone to leaking, both from the individual cells beneath the shoe / pad segments, and the
lines and fluid transfer hoses. This style had the cylindrical steel drum keyed / splined into the inside of the magnesium-aluminum
alloy wheel, and rotated with it. A drum shaped cylinder was secured to the strut end of the axle to a backing plate flange,
with the bladder cells secured to the outer periphery. The shoe / pad segments were secured to the outer surface. When the
brakes were applied, fluid expanded the circumferential bladders, pressing the shoe / pad segments outward and into contact
with the drum lining.
Invaders equipped with disc brakes all had the 3-piston calipers
and a single disc. The caliper unit was secured to the strut backing plate, usually ahead of the strut. Because the backing
plate had many holes drilled in it, the caliper could be mounted in practically any position; leading, trailing, or across
the bottom arc of the disk / rotor. The rotor ( disc ) was keyed to the inside of the wheel, and was semi-floating. Any anti-skid
components would be ahead of the brake feed lines and modulate the flow and pressure supplied to the brake assembly.
With the B-26K, a circumferential array of small pistons were
used. This brake system definitely used an anti-skid system. Combined with the Hamilton Standard 43E60 High-activity
reversible pitch propellers and those high output R-2800's, that airplane could really haul down to a stop in a hurry.
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Internal Expanding Shoe Brakes
( Earlier A/B-26 Invaders )
Consisting of four parts: brake frame,
expander tube, return springs, and brake blocks
Hydraulically operated.
Each brake block is independent; no tendency to grab.
The
Expander Tube, ET, type element is an expanding configuration designed for heavy-duty brake applications and where a compact
installation envelope is required. The ET element is hydraulically actuated. It utilizes an inflatable, elastomeric tube to create forcible engagement between the friction
shoe assemblies and drum. Torque is transmitted from the friction shoes through the
support or cage frame to the non-rotating torque plate.
Originally equipped Goodrich brake units used on earlier Invaders
- Expander Tube type.
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Anti Lock Disc braking
( A/B-26 Invaders from
1945 and Civilian conversions )
Disc brakes are much
more effective at dispersing the heat produced, maintaining their effectiveness during long periods of heavy braking. They
consist of a disc or series of discs of aluminium alloy, steel, carbon or other material, gripped between pads of friction
material. These pads are forced against the discs by pistons under hydraulic pressure. Control is usually achieved by placing
a toe pedal for each brake on its respective rudder pedal. These can then be operated differentially by the pilot, giving
the ability to steer the aircraft by applying different amounts of braking on each main wheel. Applying the brakes equally
on both main units causes the aircraft to be braked smoothly in a straight line. Large aircraft may have quite a number of
discs in each brake, to get the required braking forces and heat dissipation. The combination of carbon discs and carbon pads,
known as the carbon-carbon brake, is widely favoured, because it combines light weight and the ability to operate effectively
at extremely high temperatures.
Braking effectiveness
is reduced if a wheel locks (stops rotating), perhaps leading to loss of control of the aircraft. In some instances, the wheel
may lock but the tyre may continue to rotate. If this occurs for even a few seconds, the heat generated will cause a tyre
burst, or possibly a fire. Locking of the wheels is prevented by an anti-skid unit detecting when the wheel or wheels on any
unit stop turning, and momentarily releasing brake pressure on that unit only. This gives the aircraft the ability to stop
in the shortest possible distance without loss of control, particularly when surfaces are wet or icy. The unit works in a
similar way to the anti-lock braking system (ABS) fitted to many cars.
If the wheel and tyre
overheat, the pressures generated may cause tyre bursts. Excessive tyre pressure may be prevented by a fusible plug fitted
in the wheel. This melts before the tyre becomes hot enough to blow out, and allows the tyre to deflate slowly, easing the
steering problems caused by the tyre bursting.
Below can be seen the Hytrol anti-skid
brakes and chromed landing gear struts more commonly used on the DC-6
Three piston caliper - disk brake ( Right Main Gear ) for
Invader conversions.
Consisting of:
Disk held in the wheel by teeth or keys.
Linings on either side of the disk; compressions forms braking
action
One lining is attached to axle structure, the other moves according
to hydraulic pressure.
May have multiple pistons (& therefore multiple linings)
Operation
These discs are keyed to the wheel and rotate with the wheel.
The brake piston housing, which is securely bolted to the torque plate of the aircraft
landing gear, contains from one to four brake actuating pistons,
the number depending on the type and size of the aircraft. Depressing a brake pedal forces hydraulic fluid through connecting lines to the wheel brake assembly and Into each piston
cavity. This forces the piston and the outboard brake lining
against the brake disc. Since the disc is free to move from side to side along the lugs in the wheel, it is forced against the inboard brake lining. The friction that results from this clamping of the rotating
disc between a pair of linings retards the continued rotation to
the disc and of the wheel to which it is attached.Single-disc.
The drawing
shows a cross-section of a single-disc brake. This brake, likemost
hydraulically operated disc brakes, is self-adjusting. The illustration shows a cutaway of only one piston cavity.
However, all piston assemblies of this type of brake are nearly identical.
Application
Application of the single-disc brake occurs when hydraulic fluid under pressure enters the fluid port. This pressure forces the piston to the left against spring pressure until
the spring guide contacts the flanged side of the adjusting
pin.
In moving this distance,
the piston has forced the outboard brake lining against the steel
disc, which in turn moves sideways on its keys and contacts the inboard brake lining supported by the backing plate. The brake running clearance is thus taken up by this first movement of the piston. However, to obtain full braking friction
between the rotating disc and the lining "pucks", the piston must move farther to the left. The disc must be firmly pinched between the lining "pucks". The spring guide is already contacting
the face of the adjusting pin, and the adjusting
pin is held firmly by the friction of the adjusting pin grip (9). However, pressure on the piston provides enough force to overcome this faction, allowing the piston and
pin to move farther to the left If the brakes are
held in this applied position, the lining wears away. The pin continues to move Inward slowly to compensate
for the small amount of lining wear.
Release
Each time the brakes are released, the piston
backs away from the outboard lining block. It can move until the back of the piston head contacts the face of the adjusting pin. The
piston always releases to give the same running clearance.
The self-adjusting pin grip prevents the pin from moving to the right
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Multi-disc brake assembly
( On Mark B-26K Counter Invader and Later civilian conversions
from 1955 )
( Operation - As above )
A disc brake unit comprises a disc assembly of interleaved
rotor discs and stator discs that are normally spring in one direction biased to a brake engaged condition. A hydraulically
operated first piston engages a stator disc at one end of the unit to move it axially in opposition to the spring bias to
release the pressure between the discs and thus release the brake. A non-rotatable stop adapted to contact only one intermediate
stator disc is provided to prevent movement of the intermediate disc in a direction opposite to the one direction. A second
piston engages a stator disc at the opposite end of the unit for moving the rotor and stator discs on one side of a stop into
engagement with each other and applies a modulated force to said rotor and stator discs while the first power piston is in
position to oppose the spring bias, to thereby apply slip braking torque to the output shaft
B-26K, multi piston disc brake in a circumferential array. Note
the original mounting flange ( near ) used to mount the multi-piston brake unit ( beyond ) with through bolts.
Consisting of: Typically will have multiple rotors
and pistons, stator plates, a pressure plate and a torque tube.
Wear indicators are often included in this design.
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