"Easy Steps to Design Vehicle BRAKES"
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| Representative Pictures of brake |
It goes without saying that
brakes are one of the most important control components of vehicle. They are
required to stop the vehicle within the smallest possible distance and this is
done by converting the kinetic energy of the vehicle into the heat energy which
is dissipated into the atmosphere.
Design Calculations
The calculation is followed in following steps:
1.
Vehicle
Dynamics Calculation.
2.
Brake
requirement calculation.
3.
Hardware
Calculation.
4.
Cable
operated losses.
1. Vehicle Dynamics Calculations :
There are two types of
calculations involved viz. Static & Dynamic. While calculating brake it is
assumed that you have calculated the Basic dimensioning & Weight
distribution calculations of the vehicle.
Now, Static Axle Load distribution
:
Where,
Mr
= Static rear Axle Load (kg);
M = Total vehicle Mass (kg)'
φ =
Static axle load distribution
Now, calculate
Relative Centre of Gravity Height:
H / Wb =
X
H = Vertical Distance from C of G to ground on the
level (m)
Wb = Wheel base (m)
X = relative COG height
In
actual practice when a vehicle is tried to stop, the weight transfers from Rear
to Front wheel. Thus, it needs to be considered while designing a brake.
Therefore Dynamic Front Axle load is calculated.
[(1-φ) +
(X.a)].M = Mfdyn
Where,
a
= decelration (g units)
M = total vehicle mass (kg)
Mfdyn = dynamic front axle load distribution
“a” i.e. deceleration is given in g units
and is taken as 0.3g for Normal Automotive Brakes with Handle operated lever.
The front axle load cannot be greater than
the total vehicle mass. The rear axle load is the difference between the
vehicle mass and the front axle load and cannot be negative.
2. Requirements from Brake :
Now we need to calculate the loads
which brakes should apply to complete the desired task.
Braking Force required can be
simply calculated using Newton’s 2nd Law of Motion.
BF
=
M.a.g
Where,
BF
= Total Braking force (N)
M = Total vehicle Mass (kg)
a = deceleration (g units)
g
= acceleration due to gravity (m/sec2)
Wheel Lock :
The braking force can only be generated if the wheel does not
lock because the friction of a sliding wheel is much lower than a rotating one.
The maximum braking force possible on any particular axle before wheel lock is
given by:
FA
= MWdyn . g . µr
Where,
FA = Total possible braking force on the axle(N);
MWdyn = dynamic axle Mass (kg) ;
g
= acceleration due to gravity (m/sec2)
µr =
Coeff. of friction between Road and tyre
Now we
need to calculate the Brake torque:
Having
decided which wheels will need braking to generate sufficient braking force the
torque requirements of each wheel need to be determined. For some legislation
the distribution between front and rear brakes is laid down. This may be
achieved by varying the brake size or more likely using a valve to reduce the
actuation pressure.
T = BFw
R/r
T = Brake Torque (N-m)
BFW = Braking force for the wheel (N)
R = Static laden radius of tyre (m)
r = Speed ratio
between the wheel & the brake
3. Hardware Design :
Disc Effective Radius:The effective radius (torque
radius) of a brake disc is the centre of the brake pads by area.
For dry discs it is assumed to be:
re = (D+d)/4
Where,
re = Effective Radius (m)
D
= disc usable outside dia. (m)
d
= disc usable inside dia. (m)
For full circle brakes it is:
re = 1/3
* [ (D3 - d3)/(D2 - d2
)]
Where,
re = Effective Radius (m)
D
= disc usable outside dia. (m)
d
= disc usable inside dia. (m)
Clamp
Load:
C
= T/ (re
* µf * n)
Where,
C = Brake Clamp load (N)
T
= Brake Torque (N-m)
re = effective radius (m)
µf = Coeff. of friction of lining material
on the disc material
n =
no. of friction faces
The
clamping load is assumed to act on all friction surfaces equally. For dry disc
brakes it doesn’t matter whether the brake is of the sliding type or opposed
piston. Newton’s Third Law state every force has an equal and opposite reaction
and a reaction force from a sliding calliper is the same as an opposed piston
one.
Stopping
Distance:
S = V2 /
2*g*a avg
Where,
S = Stopping distance (m)
V
= test speed (m/s)
aavg = avg deceleration for the whole stop (g units)
g = acceleration due to gravity (m/s2)
4. Cable operated Losses :
Cable losses are not
inconsiderable and vary depending on the number and angle of bends. A typical
cable supplier uses the following calculation to calculate cable efficiency:
η
= 1000 / (Ba + 1000)
Be = Total angle of bends (degree)
After this mechnical calculations,
thermal caluclations of brake is done and then component is designed in CAD
softwares like proE, Solidworks, and then Structural, Thermal & CFD
failures are analyzed in ANSYS.
