Prof. Dr.-Ing. Stefan Vöth - Hoists with safety brakes
Hoists with safety brakes
Prof. Dr.-Ing. Stefan Vöth
Cranes integrated in the logistic chain are vital elements to
ensure safety and economic efficiency of transport. So safety is a
essential factor for service of cranes in nuclear plants or
metallurgical plants. For service of ship-to-shore-cranes the
economic efficiency is an additional dominating aspect.
Holding brake
Aligning coupling
Starting point
Due to instationary service hoists are dynamically
loaded structures. Especially for the case of
emergency-off of safety oriented hoists with a
safety brake on the rope drum disc high dynamic
internal forces occur in the drivetrain [1, 2, 3]. Thus
exists the risk of component failure, which can be
observed in practical applications. The failure of a
component, especially the hoist gearing, results in
consequences relating safety and availability and is
to be prevented for many cases of service urgently.
Concerning hoists of cranes two approaches for
planning and manufacture may be relevant. Serial
hoists are planned and built for greater number of
pieces. Hereby serial hoists as a system can be
planned and checked in detail before market
introduction. Open winches in most cases are a
system configured for a singular crane plant. They
cannot be planned as detailed as a serial hoist. In
addition intensive experimental tests for example
over planned lifespan are not possible. Thus a target
oriented approach in planning of open winches is of
special relevance.
Hoist structure
Generally hoists consist of a drive train, to the ends
of which loads are applied: At one end the motor
and the brake are located, at the other end the load
is attached (picture 1). Relative transparent
relations concerning the dynamic behavior of the
hoist in different service conditions result out of
this.
At safety oriented hoists as in ship-to-shore-cranes,
in nuclear plant cranes and in cranes for
transporting molten metal this looks a little bit
different. To cover a rupture of the drive train an
additional safety brake is located on the board disc
of the rope drum mostly. Thus a load can be applied
in the middle of the drive train [1]. In comparison
to the general hoist structure a modified dynamic
behavior of the hoist in general and the elastic drive
train especially is the consequence.
Aligning coupling
Motor
A main assembly of every crane is the hoist. Following is
shown, which internal forces occur in the components of a hoist
with safety brake and are to be considered while dimensioning.
In addition a concept for the design and the control of the
braking system is shown in order to reduce the internal forces in
hoist components of the rotational part of the drivetrain as
couplings and gearing.
Gearing
Rope drum
Rope drive
Load
Picture 1: Hoist
Brakes
Safety oriented hoists comprise more than one
brake.
Analogous to a classical hoist structure a service
brake is located on the axis of the fast running
motor shaft. As this brake in today’s electrically
braking systems is hardly used as a stopping brake,
this brake may also be named as holding brake.
In safety oriented applications the load also must be
suspended in case of rupture in the drive train.
Therefore a safety brake is located on the board
disc of the rope drum, which is at the load oriented
end of the rotating part of the drive train.
As for the case of service a stronger or a divided
braking action is to be implemented, an additional
brake may be installed on the axis of the fast
running motor shaft. This additional brake is not
necessary for general braking or for holding of the
load typically.
After switching all brakes get into action after a
certain dead time. The braking torque is increasing
according to an exponential function. The final
braking torque depends on the frictional conditions
within the brake.
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Prof. Dr.-Ing. Stefan Vöth - Hoists with safety brakes
Braking process
Switching procedure
It is task of brakes in hoists, if required, to stop the
hoist within a certain time or without exceeding a
certain further hoisting distance or lowering
distance. Subsequent the hoist is to be hold in the
reached position in the first instance.
The actions of motor and brakes during a braking
procedure are not permanent. In fact a sequence of
omitting and adding loads on the drive train occurs.
Especially the case of emergency-off is considered
here with following chronological scenario: After
activation of emergency-off and a dead time ∆tM the
motor torque is dropped out. Parallel the brakes get
into action. Takes the brake application more time
than the drop out of the motor torque, exist the dead
times for the service brake ∆tBB and the safety brake
∆tSB (picture 3).
During deceleration a braking torque must be
delivered by the brake or the brakes, which consists
of three fractions typically:
Braking torque fraction out of static load
torque of suspended load including load
suspension device and load attachment
device,
Braking torque fraction out of braking
torque for the deceleration of the rotational
masses,
Braking torque fraction out of braking
torque for the deceleration of the
translational masses.
Amount and direction of action of these three
fractions comply with the considered load case,
which can be understood as the transition between
two service conditions. The service conditions
respectively the transitions between them can be
visualized in the four-quadrant-diagram (picture 2).
Especially can be distinguished:
Hoisting
t
Off
Motor
Lowering
Emergency Off Event
∆tM
∆tBB
On
t
Off
Service brake
∆tSB
On
t
Off
Safety brake
Holding service (v=0),
Hoisting service and lowering service
(v≠0),
Acceleration and deceleration (a≠0),
Load condition, in extreme cases dead load
plus rated capacity and dead load.
The load condition itself is not indicated in the
four-quadrant-diagram.
a
x
v<0, a>0
Braked lowering
x
v>0, a>0
Accelerated hoisting
Picture 3: Chronological scenario for switching
processes following emergency-off
Reference system
For a closer look on the behavior a loss-free, partly
redundant hoist with safety brakes is considered
(picture 4).
Central element of the hoist is the gearing. The load
is suspended by a load attachment device and a
rope drive with 8/2 reeving. Both ropes are running
onto a drum each, which are coupled with the
gearing output shafts. On the board disc of each
rope drum a safety brake is located. The hoist is
driven by two motors which are connected to the
gearing input shafts. On the motor shafts axis a
service brake is located each.
The reference hoist is described by following data:
v
x
v<0, a<0
Accelerated lowering
x
v>0, a<0
Braked hoisting
Picture 2: Four-quadrant-diagram
Motor speed
Hoisting speed
n1=1500min-1
vH=45m/min
Mass motor shaft
Mass rope drum shaft
Mass load attachment device
Mass SWL
θ1=20kgm2
θ2=500kgm2
mLAM=10t
mSWL=52t
Radius rope drum
Gearing ratio
r=0.5m
iG=26.2
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Prof. Dr.-Ing. Stefan Vöth - Hoists with safety brakes
Rope drive ratio
iS=4
Service brake torque
Dead time service brake
Safety brake torque
Dead time safety brake
MBB=5.8kNm
ttotBB=0.4s
MSB=130kNm
ttotSB=0.1s
hoist, which disposes of a fast acting safety brake
and a slow acting service brake. Emergency-off
means, that energy supply is cut off spontaneously
and all components react accordingly. The motor
torque is omitted and the brakes apply
mechanically.
Gearing stiffness
Clearance drum coupling
c=4e4Nm/rad
s=3°
The special case of a blocked load in hoisting
service (snag) is not considered here for a start [2].
Holding brake
Rigid body kinetics
Holding brake
According to a rigid body approach the acceleration
of the drivetrain α1 results as quotient of the sum of
accelerating torques ΣMred and the sum of rotational
masses Σθred, both values for example reduced to
the motor shaft:
α1 =
Rope drum
Safety brake
Rope drive
∑M
∑θ
red
red
Safety brake
Rope drum
Aligning coupling
Gearing
Rope drum coupling
Rope drum coupling
Motor
Aligning coupling
Motor
α1
∑Mred
∑θred
Rope drive
Load
Picture 4: Reference system
Load cases
The hoist underlies in service different load cases,
described by following parameters:
Concerning the direction of movement
holding, hoisting and lowering can be
distinguished.
Concerning the variation of speed
constancy, acceleration and deceleration
can be distinguished.
Concerning the load suspended at the rope
drive loads from dead load (load
attachment device) to full load (load
attachment device + safe working load)
may occur.
Concerning the internal forces are switching
processes of interest. In doing so changes between
following service conditions can occur:
Suspended load
Hoisting
Lowering
Service-stop
Emergency-stop
Emergency-off
Acceleration of drivetrain
Sum of accelerating torques, reduced to
motor shaft
Sum of rotational masses, reduced to
motor shaft
The accelerating torques result from the motor
torque MM, the torque of the service brake MBB, the
torque of the safety brake MSB and the torque out of
the load ML. The rotational masses result from the
rotational masses of the drivetrain θ1 and θ2 as well
as from the translational masses of the load mLast
including the load suspension device and the load
attachment device mLAM.
For the hoist represented as a rigid body model the
behavior of load speed over time can be calculated.
As a result for example the speed over time for
different mechanical braking scenario out of
hoisting/lowering the dead load are gained
(picture 5).
With the acceleration of the drive train α1, the
motor torque MM, the braking torque of the service
brake MBB and the rotational mass of the motor θ1
results for the relevant gearing input torque MG
(picture 6):
M G = M M − M BB − θ1α1
α1
θ1
MG
MM
MBB
Acceleration of drivetrain
Rotational mass of motor
Gearing torque
Motor torque
Braking torque service brake
Following load cases are considered, which will
lead to high internal forces in the drivetrain by
trend. Considered is the case of emergency-off for a
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Prof. Dr.-Ing. Stefan Vöth - Hoists with safety brakes
50
7,0s; 2633mm
Hoisting
Hoisting, Dead load
40
30
0,4
-10
0,6
0,8
Time t in s
1,0
566mm
0,2
466mm
0
359mm
∆tBB
296mm
267mm
10
302mm
Hoisting speed vH in m/min
20
As observed the deceleration of the hoist occurs
quickly. This is caused mainly due to the braking
by the high load. Thus the deceleration process is
finished as fast that the service brake (holding
brake) with greater dead time in general is not or
fairly not coming into action anymore. Firstly the
torque for braking the motor mass is supplied by
the load and the safety brake and transferred to the
motor mass via the drive train.
Assumed
simultaneous action of the safety brake and
switch-off of the motor,
braking torque build up according to the
character of a jump function,
a delayed action of the service brake
(holding brake) in comparison to the safety
brake and
a loss free drivetrain,
1,2
∆tSB
-20
-30
Lowering
-40
Lowering, Dead load
-50
the maximum relative gearing input torque for
braking with the safety brake out of hoisting (+) or
lowering (-) reaches the level
-60
M G rel max = φ5 (MF(LF ± BFSB ) − LF ) + LF
-70
MG rel
Picture 5: Speed characteristics for braking the
dead load out of hoisting/lowering
MG rel max
MF
MG
LF
BFSB
θ1α1
ω1
θ1
MM
MHB
Picture 6: Torques acting at the cut free motor shaft
Elastic body kinetics
The rigid body approach does not consider
elasticity and clearance in the hoist system.
Accordingly it is of interest to investigate the
influence of these properties. Therefore the rigid
body model is expanded by adding the elasticity of
the gearing and the clearance in the rope drum
coupling.
Braking during hoisting
Examinations show the special relevance of the
load case emergency-off out of hoisting the
maximum load.
φ5
Gearing input torque relative to the
static load torque out of maximum load
Maximum value of MG rel
Mass factor: rotational mass of motor
relative to total rotational mass of
drivetrain reduced to motor shaft
Load factor: lifting load relative to
lifting capacity
Braking factor safety brake: braking
torque safety brake relative to static load
torque out of full load
Dynamic factor for mass forces out of
drives according to BS EN 13001-2
Here the torque jump resulting out of the change of
service condition according to the rigid body model
is assessed with the dynamic factor for drives φ5
corresponding EN 13001-2 [4]. For braking
maximum load out of hoisting results
M G rel max = φ 5 [MF (BFSB + 1) − 1] + 1
Hence the quasi static gearing input torque is:
M G rel = MF (BFSB + 1)
That means: In the most unfavourable case (masses
almost completely concentrated on the motor shaft
(MF=1), maximum load at load attachment device)
a maximum quasi static gearing input torque of
braking torque safety brake plus static load torque
can occur:
M G rel = BFSB + 1
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Prof. Dr.-Ing. Stefan Vöth - Hoists with safety brakes
The dynamic peak torques (picture 7) can occur, as
far as they are supported by the static load, braking
torque and inertia torque. Details regarding this are
to be determined by an elasto-kinetic analysis. It is
obvious for the considered load case that internal
forces resulting in the drive train are a multiple of
the static holding torque.
8
8,0
7
7,1
6,3
6
5,9
5,6
φ5=1,5,MF=1
5
Analyses show that especially the load case of
emergency-off out of lowering the dead load is of
interest.
Emergency-off immediately initiates switch-off of
the motor and activation of the safety brake. For the
mentioned data a maximum relative gearing input
torque of MG rel max=8.9 is calculated (picture 9).
That means the gearing torque is factor 8.9 higher
than the maximum static loading of the drive train.
0
4
3,0
32,8
2,5
2,6
2,4
2
Dynamic factor φ5=2,0, Mass factor MF=0…1
2,2
Dynamic factor φ5=1,5, Mass factor MF=0…1
Dynamic factor φ5=2,0, Mass factor MF=0,90…0,95
1
Dynamic factor φ5=1,5, Mass factor MF=0,90…0,95
0
φ5=1,5,MF=0
-0,5
-1
1,0
1,5
2,0
-4
φ5=2,0,MF=0
-1,0
2,5
3,0
3,5
Relative braking torque safety brake MSB/Mst
Picture 7: Maximum relative gearing torque
For a braking factor of the safety brake of BFSB=1.7
results a relative peak gearing torque assessed
according to BS EN 13001-2 of MG rel max=2.5…4.2.
For BFSB=3.3 this value reaches a level of
MG rel max=3.9…7.2. That means the gearing torque
is about factor 7.2 higher than the maximum static
internal force in the drive train.
A special illustration for φ5=1.0 (rigid body
approach) shows picture 8.
5
Rel. gearing torque MG rel/Mst
4,5
4,1
4
MF=1
3
MF=0,9
2
2,3
2,0
1,8
1
MF=0,5
1,0
0
1,0
1,5
2,0
2,5
3,0
3,5
Relative braking torque safety brake MSB/Mst
Picture 8: Relative gearing torque
With the rigid body model (without consideration
of a dynamic factor) the maximum relative gearing
input torque, that is the gearing input torque relative
to the static gearing input torque out of maximum
load, can be calculated. For the typical hoist design
here it reaches a level of MG rel max=2.6, for special
designs it may reach a level of MG rel max=4.4. That
means the gearing torque is factor 4.4 higher than
the maximum static loading of the drive train.
Rel. Gearing Torque MG rel/MSt
Maximum relative gearing torque MG rel max/Mst
7,6
φ5=2,0,MF=1
Braking during lowering
-8
Time t in s
0.0
0.25
0.50
0.75
1.0
Picture 9: Relative gearing torque according to
elasto-kinetic hoist model
During lowering the hoist is driven by the load,
which is hold in steady state condition by the
motor. When the safety brake gets into action, the
stoppage is executed very fast for this case as well.
On one hand this is caused by the low load level,
dead load. Assuming clearances in the drive train
(in gearing and/or couplings) it is expected
furthermore, that a flank change will occur. During
this the motor side masses and the braked load side
are uncoupled. Respectively the motor side masses
need not to be decelerated.
At an appropriate constellation the load side will
stand still before running through the clearance is
finished. In this case after running through the
clearance a shock will occur. The motor side
pitches on the standing load side. Toothed wheels
and bearings in the gearing are loaded significantly
by this shock. A special shock load may occur to
the bearings of helical gearings. In this case the
shock is led in axial direction of the shafts and with
it on the roller bearing acting as fixed bearing.
Gearing loading
From the calculations maximum internal gearing
torques much higher than according to static or
rigid body approaches can be derived. Especially in
the gearing such shock-like internal loads appear
after running through clearances in relation with
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Prof. Dr.-Ing. Stefan Vöth - Hoists with safety brakes
Hertzian contacts (toothing, roller bearings). To
ensure safety such maximum internal loads must be
covered statically. To ensure durability such
maximum internal loads should not lead to
pre-damages, which would lead to fatigue under
further service loading.
Hoisting
t
Off
Motor
Lowering
Emergency Off Event
∆tM
„Intelligent“ braking
On
In zones of maximal exploitation of the strength
pre-damages of the material must be prevented.
Shall the gearing not be dimensioned too large, the
appearing internal forces are to be restricted. For
the given hoist structure approaches to reduce the
maximum values and amplitudes of the internal
forces are demanded.
As measure to reduce peak values and amplitudes
of the internal loads is considered:
Synchronous and balanced action of all
brakes participating in the braking process,
here the service brake and the safety brake.
Main reason for high internal forces in the drive
train during a safety braking process is following
situation: The maximum of the kinetic energy to be
reduced is concentrated in the masses on the axis of
the fast running motor shaft: masses of motor,
coupling and braking drum/braking disc. Braked
will be at first at the board disc of the rope drum
and by the load. So the brake torque is not induced
at the location of demand. A significant part of the
brake torque must be led from the location of
induction to the fast rotating masses. To prevent
this torque put through the gearing it makes sense,
to bring the service brake into action synchronous
to the safety brake.
This leads to a direct participation of the service
brake in the braking process. This ideally results in
a switching scenario with dead times of the service
brake and the safety brake of ∆tBB=∆tSB=0s.
Requirement is a holding of the motor torque until
both brakes get into action (picture 10).
t
Off
Service brake
On
t
Off
Safety brake
Picture 10: New chronological scenario for
switching
processes
following
emergency-off
Remains the question with which amount of torque
the safety brake and the service brake should act.
Favourable would be braking in a way that the
quasi static internal torque before braking is still
present during braking. Hereby at the beginning of
braking a jump in the internal torque during
transition from “hoisting/lowering” to “decelerated
hoisting/lowering” is prevented. Likewise at the
end of braking a jump in the internal torque during
transition from “Decelerated hoisting/lowering” to
“holding”
is
prevented.
Assuming
these
requirements given for the structure of the reference
hoist following braking factors for the safety brake
and the service brake for the braking out of hoisting
(-) or lowering (+) are calculated:
*

θ ges
∆ω 
BFBB = m  LF + MF *


M
st ∆ t 

BF SB = m (1 − MF
BFBB
BFSB
LF
MF
θges*
Mst*
∆ω
∆t
)
*
θ ges
∆ω
M
*
st
∆t
Braking factor service brake: braking
torque service brake relative to static
load torque out of full load
Braking factor safety brake: braking
torque safety brake relative to static load
torque out of full load
Load factor: lifting load relative to
lifting capacity
Mass factor: rotational mass of motor
relative to total rotational mass of
drivetrain reduced to motor shaft
Total rotational mass of drivetrain
reduced to motor shaft
Static load torque out of load capacity
reduced to motor shaft
Difference of motor shaft Angular
frequency
Braking time
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Prof. Dr.-Ing. Stefan Vöth - Hoists with safety brakes
Brake factors service brake BFBB and safety brake BFSB
2,5
Hoist with balanced braking:
Brake factors braking out of hoisting
BFBB+BFSB
BFBB
BFSB
2,0
1,5
1,0
0,5
0
0
10
20
30
40
50
Load in t
Picture 11: Braking factors for braking out of
hoisting
For the reference hoist the brake factors and their
sums are calculated as shown for braking out of
hoisting (picture 11) and out of lowering (picture
12).
For braking out of hoisting the safety brake has to
be applied to a little account only (BFSB=5%-12%).
The service brake has to deliver a significant torque
under partial load. With increasing loads up to full
load the braking torque of the service brake is
decreasing continuously (BFBB=92%-8%).
Brake factors service brake BFBB and safety brake BFSB
2,5
Hoist with balanced braking:
Brake factors braking out of lowering
BFBB+BFSB
BFBB
BFSB
2,0
For the higher internal loads occurring especially
due to emergency-off [3] the hoist may be not
dimensioned reasonably and efficiently. Assuming
a corresponding hoist concept this also applies for
emergency-stop, a load case occurring more often.
Accordingly measures have to be considered in
order to reduce internal loads induced to the drive
train. The internal loads in the drivetrain are
reduced especially by “intelligent braking”. Ideally
the braking process is designed in a way, that
during braking in the drivetrain between motor and
safety brake the torque during static hoisting is
present also. Hereby the maximum values as well
as the amplitudes of the internal forces are reduced
significantly. Suitable measures to be applied are:
Reduction of clearances and increase of system
elasticity: As result shocks can be reduced and
absorbed, as well as internal loads are reduced in
connection with system damping.
Minimisation of dynamic effects: Following BS
EN 13001-2 [3] this may be realized by little
clearance and a gradual implementation of the
braking torque.
Reduction of mass factor MF: By a small share of
the motor mass in relation to the total mass of the
drivetrain the torque put through the gearing is
reduced.
Minimisation of brake factor BFSB: A small
braking torque of the safety brake generally leads to
less braking action and reduced internal forces.
Braking action synchronous to motor switch-off:
Is he motor moment decreasing before braking
action takes place, the drivetrain is relieved slightly.
The resulting internal forces can be prevented by
synchronity of the events.
1,5
1,0
0,5
0
Conclusions
0
10
20
30
40
50
Load in t
Picture 12: Braking factors for braking out of
lowering
For braking out of lowering the safety brake has to
deliver only a small torque (BFSB=5%-12%). The
service brake has to deliver a significant torque
under partial load. With increasing loads up to full
load the braking torque of the service brake is
increasing continuously (BFBB=124%-208%).
Synchronized application of safety brake and
service brake: In order to prevent torques put
through the drive train a synchronized application
of both brakes is inevitable. As a result the collision
of the non-braked massive drive side mass (motor)
and the braked load side mass (rope drum) is
prevented. Corresponding shocks in assemblies
with clearances as gearing and rope drum coupling
are reduced. In typical hoist structures the dead
time of the safety brake is significantly lower than
that of the service brake. An expandation of dead
time of the safety brake in most cases cannot be
accepted. Accordingly a suitable approach is to
shorten the dead time of the service brake [4].
Balanced braking torque of safety brake and
service brake: For adjusting the torques in the
drive train defined braking torques at safety brake
and service brake are required. Advisable is the
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Prof. Dr.-Ing. Stefan Vöth - Hoists with safety brakes
balancing of both braking torques according to the
energies to be dissipated at the locations of brakes.
These braking torques depend on the service
condition and the suspended load. Brakes with
controllable torques are applied ideally. For cranes
they are not state of the art today. Instead of the
step less adjustment of torques a stepped
adjustment of braking torques may be considered.
This is realized by a parallel arrangement of several
smaller brakes at one braking location. Hereby an
approximation of the ideal condition is achieved.
Low motor mass.
Low braking torque at safety brake.
Further measures
consideration.
The hoist has to cope with those service situations,
especially in zones of Hertzian contacts as toothing
and roller bearings of gearings. Therefore for
critical systems beside the classical measures to
reduce internal loads as
Increase of elasticity and damping in the
drivetrain.
Reduction of clearances, especially in
drum coupling and gearing.
be
taken
into
It is proposed to evolve the braking systems of
hoists to “intelligent braking systems”, which are
able to anticipate the service condition.
Structural and control elements of such a braking
system are:
Summary
In the logistics chain integrated crane plants are
essential elements to ensure safety and economic
efficiency of the transport. An essential assembly of
every crane is the hoist. Under appreciation of
dynamic processes it can be calculated, that in case
of emergency-off significant torque peaks in
gearings of hoists can occur. Here relative torques
of about MG rel max=890% were determined. Those
peaks are not covered by a rigid body analysis and
subsequent also not by a dimensioning according to
BS EN 13001-2 [4] basing on such a rigid body
analysis.
should
Braking action synchronous to motor
switch off,
synchronous action of safety brake and
service brake and
Balanced action of safety brake and
service brake.
Literature
[1]
[2]
[3]
[4]
[5]
RWTÜV Schriftenreihe, Heft 8, Krane,
Bemessung und Sicherheit, 1981
Vöth: Safety Systems for Container Cranes,
17th ITI Symposium, Dresden, 2014
Schmeink: Dynamische Beanspruchung von
Hubwerksgetrieben,
Tagungsband
22.
Internationale Kranfachtagung, Magdeburg,
2014
EN 13001-2: Crane safety, General design,
Part 2: Load actions
Römer: Difference between dynamic and
static coefficient of friction, Port Technology
International, 56. Edition, Winter 2012,
P. 49-51
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