Nexen
CLUTCH AND BRAKE SELECTION PROGRAM
This program is designed to save you time and effort when selecting Nexen Pneumatic Power Transmission Components. To obtain the maximum benefit from this program you should thoroughly understand this document. Hyperlinks are provided to link to in-depth information at the end of the document. To return to your previous location use your browsers back button.
This program can only obtain meaningful results if the input
data accurately reflects the characteristics of the application. While it may
not always be easy to obtain accurate information, a full understanding of the
data input criteria is essential. In the text that follows, a definition of
each input is explained. There may be more information than is required for
your particular application since these definitions cover all sections of this
program.
1.
Operating
Rotational Speed (revolutions per minute): This refers to the shaft speed that the product will be mounted
on
2.
Transmitted
Power (horsepower or kilowatts): This is the mechanical power that the product must be able to
transmit without slipping at the specified rotational speed, not the horsepower
of the driver
3.
Rotational
Inertia Properties (pound-feet2 or
kilogram-meter2): This is the total reflected
inertia transmitted to the clutch or brake. If this value is unknown select
light, medium, or heavy from the list. The selections only function when the
inertial value input box is blank otherwise the inertial value is used. For
assistance in calculating the actual inertia of your application see the
inertia section at the end of this document More
Information
4.
Desired
Load Response Time (seconds): This is the length of time it takes the clutch to bring the
load up to the speed of the driver or the brake to bring the load to a stop
(measured from the instant the control valve opens). In many applications, the
load response time is not critical, and a value in the range of 0.5 to 2.0
seconds may be used. In high inertia applications, this value becomes more
important. Cutting your chosen time in half will double the clutch or brake
starting torque requirements, while doubling the time will reduce the clutch or
brake starting torque requirements by half. This has a major impact on unit
size and cost.
5.
Number
of Engagements or Applications Per Minute: This is the cycling rate for the application. A value of one
may be entered for this input if the application is non-cyclic (less than one
engagement per minute). The higher the cycle rate the greater the clutch/brake
requirements. Consult Nexen if you are planning to use a spring-engaged product
in a cyclic application since spring fatigue can lead to clutch or brake
failure.
6.
Actuating
Air Pressure (pounds per inch2 or
bars): This is
the air pressure the clutch or brake is to operate at, generally in the range
of 15 to 80 psi. Lower pressures will lead to the selection of larger products,
but selecting a product at a high pressure will leave less margin for
application error and shorter product life More
Information
7.
Static
load during engagement (percent): This is the portion of the static load torque that is present
during start-up. This feature is designed to accommodate applications in which
the clutch does not have to transmit the full load torque until after the load
has been brought up to speed (i.e. fans and pumps).
8.
Required
Holding Torque (foot-pounds or Newton-meters): In the case of a brake or clutch-brake
selection, this refers to the external torque accelerating the load that the
brake is trying to stop (i.e. elevated loads or other systems where there is
stored energy). In many applications, the holding torque will be zero More Information
9.
Web
Speed (feet per minute or meters per minute): The speed of the web (material being converted) during normal
production running. This does not include web-up speed, inching, jogging, etc.
10.
Web
Tension (pounds or Newtons). This is the total tension to be maintained on the web during
the unwind or rewind process. If the total tension is unknown and you have one
of the material groups listed below the web tension input box, leave the web
tension box empty and check the appropriate material type, you will be asked
for more material information on the next page.
11.
Web
thickness (Basis weight, mils, microns, points, grams per meter2, AWG):
Basis weight: The weight in pounds based on a 3000 ft2 ream
Mills: One mill = .001 inches
Microns: One meter = 100,000 microns
Points: One point = .001 inches
AWG: Average wire gage a USA based measurement
If the web tension box has a value in it, the material selection
section list has no effect
12.
Web
Width (inches or millimeters): The total web width
13.
Minimum
roll diameter (inches or millimeters): The outside diameter of the roll core
14.
Maximum
Roll Diameter (inches or millimeters): The diameter of the full roll
15.
Ratio
of Clutch or Brake Speed To Roll Speed: Typically, this is 1 to 1 but in the case of a tension control
clutch some over-drive (5-10%) is needed to prevent lock up or stick slip
conditions as the roll approaches its full diameter. Another possible scenario
would be a clutch or brake mounted on a jackshaft that is not driving the
product roll shaft at a 1 to 1 ratio.
16.
Maximum
Available Air Pressure (pounds per inch2 or bars). The maximum air pressure available from
the control system
17.
Differential
Engagement Speed (rpm):
This is the maximum speed that the tooth clutch will be engaged at. Note: Tooth
clutches have no friction interface to gradually bring the application up to
speed and rely on the meshing of teeth to transmit the power. Therefore care must
be taken to not use tooth clutches in applications that could be damaged by
engagement shock or cause clutch slipping on engagement since this could damage
the tooth interface. Application inertia and the differential speed are the key
determining factors in determining acceptable engagement speeds.
The final report will list in order of increasing torque capacity, the clutches or brakes which meet the application requirements. Many of the product attributes are fixed and do not change based on the application (like torque, maximum rpm), and others (like facing life, air consumption) do. The accuracy of the variable factors will be highly dependent on the accuracy of the input data.
The details button will show a list of all the clutch/brake candidates evaluated for the given conditions. The entries with a green check mark met the input criterion and are listed on the search results page. Rejected candidates are also listed, and if you click on the plus sign to the left the candidate, it will expand to show all of the performance attributes that were passed or failed. This can be very helpful when an expected product does not appear in the results.
The performance summary for each selection shows three ways of
connecting the control valve to the clutch: (1) an accumulator placed in the
air line between the control valve and the clutch, (2) a flow control placed
between the control value and the clutch, and (3) the control valve alone
connected to the clutch. In each case, it is assumed that the total length of
interconnecting tubing between the control valve and the clutch is one foot.
There are two columns shown for each control valve configuration;
one gives the performance of the selected clutch with a new friction facing,
the other gives the performance with a fully worn friction facing. The condition
of the friction facing affects clutch performance because, as the facing wears,
the chamber volume increases, thus increasing the clutch response time. The
accumulator and the flow control are sized by the program such that, with a
fully worn friction facing, both configurations have the same performance and
provide the load response time specified by the user. of course, with a new
friction facing, the load response time is faster, but the performance for the
two configurations is not the same. The reason for this is that, in the
accumulator configuration, the change in system volume due to facing wear is a
smaller percentage of the total volume. Therefore, the use of an accumulator to
control load response time will minimize the variation in clutch performance
caused by facing wear.
Results Definitions
1.
Design
Torque (inch-pounds or Newton-meters): The maximum torque output of the clutch or brake.
2.
Engagement
Pressure (pounds
per square inch or bars): The
minimum pressure at which the clutch or brake will engage and begin delivering
torque
3.
Maximum
Pressure (pounds per square inch or bars): The maximum pressure that the clutch or brake can be operated at
without degrading its life
4.
Work
Capacity (horsepower hours or kilowatt hours): The work that the friction facings can do
before they wear out. More Information.
5.
Maximum
rpm (revolutions per minute): This is the maximum rpm that the clutch or brake can withstand
with out mechanical failure or significant reduction in bearing life
6.
Max
Load Speed During Stop (revolutions per minute): In some applications there is stored energy
in the form of springs, pressure, or elevated loads. If the application is
operating a given speed and the brake is signaled to stop, it is possible for
this stored energy to accelerate the load to a higher rpm before the braking
effect takes place. It is important that the maximum allowable speed of the
brake and application is not exceeded during this short interval.
7.
Air
Chamber Volume New and Worn (cubic inches or cubic centimeters): Piston or diaphragm movement operate
the clutch or brake, and the enclosed area is referred to as an air chamber.
When the friction facings are new the volume is at its minimum and when the
facings are worn the volume is at its maximum. The exception to this rule is
some spring-engaged brakes that have the same volume over their life time.
8.
Friction
Facing Area (square inches or square centimeters): This is a measure of the contact area
between the friction facing and the surface it engages. This value is useful
when looking at the peak input rate since facing
materials have limits as to how much energy they can withstand without being
damaged.
9.
Flow
Coefficient (Cv) of Control Valve: A measure of the control valves ability to deliver air flow
10.
Flow
Coefficient (Cv) of Control Valve plus Flow Control: By adding a flow control valve to the control
valve, load response times can be extended to a few seconds with no loss of
torque (soft start). The additional slippage will reduce facing life.
11.
Accumulator
Volume (cubic inches or cubic centimeters): This is the recommended volume to achieve the desired load
response time with the recommended control valve. Accumulators are often needed
to extend the load response time beyond a few seconds (soft starts). Soft
starts are often used to protect the machinery or product from shock damage or
can allow the use of a smaller drive motor because the peak torque requirement
is reduced. The additional facing slippage will reduce facing life, and the
accumulator volume will significantly increase air consumption.
12.
Load
Response Time (seconds): The
amount of time it takes to bring the load up to speed or a stop
13.
Clutch
or Brake Response Time (seconds): The amount of time between the signaling of the control valve
and the beginning of torque generation
14.
Ratio
of Peak Torque to Static Torque (inch-pounds or Newton-meters): This compares the torque required by the
driver (often a motor) to bring the application up to speed in the specified
time to the normal running torque. More
Information.
15.
Peak
Input Rate (% of Max): This
is the maximum energy input rate of the application compared to the clutch or
brakes capacity expressed as a percent. Note: Exceeding 0.9 thermal horsepower
per square inch of facing area can damage the facing material. Disc brakes can
have a peak input rate of up to 2 thermal horsepower per square inch of facing
area because the swept disc area allows for some cooling. Smaller discs and
multiple calipers will reduce this value.
16.
Energy
per Cycle (% of Max): This
is the amount of thermal energy generated with each cycle
17.
Maximum
Air Consumption (Cubic feet per minute or Cubic Meters per minute): Amount of compressed air used per minute
based on the input parameters More Information
18.
Friction
Facing Life (cycles or hours): The average facing life base on the application parameters
19.
Roll
Speed (revolutions per minute): The material roll speed in a tension control application
20.
Brake
or Clutch Speed (revolutions per minute): In tension control applications where the clutch or brake is not
mounted directly on the roll shaft and driven at some ratio relative to the
roll shaft
21.
Thermal
Dissipation (% of Max): This
is a measure of the applications thermal requirements compared to the thermal
capability of the clutch or brake. The lower the number the cooler and longer
the clutch or brake will operate.
22.
Required
Thermal Dissipation (horsepower or kilowatts): The minimum thermal dissipation that a
clutch or brake requires for this application
23.
Effective
Cooling Speed (revolutions per minute): The effective cooling speed is similar to an average speed used
to perform thermal dissipation calculations
Air Consumption in Depth:
The compressed air requirements for pneumatic clutches and
brakes operating in cyclic applications are generally less than one would
expect, given their capabilities. They operate on static air pressure, that is,
they only consume air when cycled on and off. The performance summary gives the
maximum air consumption for each control valve configuration. For a given cycle
rate, the maximum air consumption occurs when the friction facing is fully
worn, because the chamber volume is at its maximum (see the performance summary
for a comparison of chamber volumes under new and fully worn facing
conditions). Notice that the air consumption for the first control valve
configuration is greater than for the other two, which are always equal. This
is due to the additional system volume created by the accumulator.
Air Pressure in Depth:
In some applications, it may not be advisable to operate a
clutch or brake at its maximum recommended pressure, even if this pressure is
available. For example, suppose you need a friction clutch for a certain
application and have a 70 psi air supply available, so you enter an actuating
air pressure of 70 psi when you run the selection program. Let us further
suppose that one of the clutches selected by the program is an Air Champ II
Model M-800 Clutch, but because of a shaft size limitation, or some other reason,
you want to use a bigger clutch, such as an H-1000. You can force the program
to select an H-1000 Clutch by reducing the actuating air pressure from 70 psi
to, say, 35 psi. By doing this, we are not merely manipulating the input data
to obtain the clutch selection we wanted in the first place, rather, the
program is forcing us to use a lower pressure--if we want to select an H-1000
Clutch. By operating the clutch at the lower pressure, a number of benefits are
realized, such as longer bearing life, longer friction facing life, lower peak
torque, lower peak input rate, and lower air consumption.
It seems obvious that one should not operate a clutch or brake
at a pressure that is much lower than the pressure for which the selection is
made, but operating at a much higher pressure can also lead to problems. For
example, it is possible to have a friction clutch that works just fine in an
application at 35 PSIG, and then someone turns the pressure up to 70 or 80 psi.
This action could overload the driver and/or lead to clutch failure due to
excessive peak input rate, even though all the other operating conditions are
unchanged.
In terms of operating pressure, another consideration applies to
combination clutch-brakes having a spring-engaged brake. When the user
specifies desired starting and stopping times for the load, the program
probably will not be able to exactly achieve each of these at the worst-case
condition (fully worn friction facings). Depending on the pressure used, one of
these times will be equal to the starting or stopping time requested and the
other will be faster. The reason for this is that there is only one air chamber
involved (the brake is spring-set) and we cannot easily independently control
the clutch and brake response times like we can with a clutch-brake having an
air-engaged brake (two air chambers). In many cases, however, it is possible to
have equal starting and stopping times (if this is really required) by doing a
trial-and-error adjustment of the pressure input to the program. For example,
an examination of the torque vs. pressure graphs for the FMCBES units reveals
that, as the operating pressure increases, the clutch torque increases relative
to the brake torque. Furthermore, at higher pressures, it takes longer for the
air pressure to drop to the point where the brake torque takes effect. The net
result of these effects is as follows:
To increase the starting time relative to the stopping time,
decrease the air pressure.
To decrease the starting time relative to the stopping time,
increase the air pressure.
Facing Life in Depth:
Friction facing life is a function of the total amount of work
done at the friction interface and is determined by the surface area of the
friction interface, the facing thickness, and the wear rate of the facing
material. In addition, the wear rate of the facing material increases as the
interface temperature increases. The program considers all these factors when
computing friction-facing life.
The performance summary shows the expected friction facing life
for the selected clutch or brake operating in the application described by the
user. Facing life is given in cycles for power transmission applications and
hours of operation for tension control applications. For power transmission
applications, facing life is computed for all three control valve
configurations, there may or may not be much difference among the values,
depending on the application and the clutch or brake selected. Extended start
times beyond a few seconds will often require the use of a flow control valve
and/or an accumulator and due to the additional slippage facing life will be
shortened.
Holding Torque:
For both air-engaged and spring-engaged brakes, there is an
input parameter called "Required Holding Torque”. This is not the torque
needed to stop the load or the torque capacity we want the selected brake to
have--it is the torque that must be applied to the load to prevent it from
rotating after it has been brought to a stop. It can also be thought of as the
torque that is trying to accelerate the load that the brake is trying to stop.
It is encountered in applications where gravitational forces and vertical
motion are involved, such as hoists, cranes, elevators, and loaded conveyors.
Thus, if a machine is initially at rest and it is possible to disengage the
brake without the machine moving under the influence of gravity or any other
external force, the required holding torque is zero. If the required holding
torque is not zero, the brake must supply enough dynamic torque fast enough to
"catch" the load and prevent it from "running away" and
possibly exceeding the safe rotational speed of the brake, or the machine
itself. Generally, a spring-engaged brake would be used in such an application.
Note that it is possible for the initial rotational speed of the load to be
zero, provided the required holding torque is non-zero (otherwise the brake
would act as a do-nothing device). In any event, the detailed performance
summary for each brake selected shows the maximum RPM that the load will
achieve during the stop. If the required holding torque is zero, the maximum
RPM is simply equal to the initial RPM of the load.
Inertia
in Depth:
Inertia is a measure of an objects resistance to a change in
motion. When starting or stopping an application this inertia must be overcome
and requires the dissipation of energy in the form of heat. In the case of a
clutch or brake, care must be taken that it can withstand the required
application energy input or it will fail quickly. The basic calculations for
inertia in English units are:
·
In the
case of a solid shaft:
Inertia = WK2 =
w(D2/4), Where w is the weight of the object
and D its diameter.
·
In the
case of a hollow shaft or if the weight is unknown:
Inertia = WK2 =
.000681p L (D24 –
D14), Where p is the density of the material
(steel = 0.282, cast iron = 0.260, aluminum = .0924) lb/in3, L is
the length in inches, D2 is the outer diameter, and D1 is
the inner diameter.
Application components rarely consist of plain shafts only. More complicated geometries should be broken down into simpler cylindrical shapes calculated separately and then added together.
Reflected inertia is a measure of the
change in the inertia as it passes through a speed-changing device like a
reducer, pulley, or sprocket arrangement. These devices can amplify or reduce
the inertial load having a profound impact on the required clutch or brake size
therefore any speed changes between the load and the clutch or brake must be
identified. If there is a speed decrease between the clutch or brake and the
load, the reflected inertia is reduced by the square of the reduction. For
example 100 lb-ft2 inertial load turning at 58 rpm attached to a
30:1 reducer driven by a clutch turning at 1750 rpm would have a reflected
inertia of just .11 lb-ft2 (100/(30*30)) as seen by a clutch on the
input of the reducer. In cases like this the load inertia can seem to become
inconsequential but the input shaft to the reducer, coupling, and the
clutch/brake its self have inertias of their own that may be more significant
and should be added together. This concept also applies to pulley and sprocket
systems where the reduction significantly reduces the application inertia but
the pulleys, sprockets, and even the chains and belts can add a significant
amount of inertia. The selection program automatically includes the rotational
inertia of the clutch or brake in it’s calculations. The program will estimate
the rotational inertia of the load if the user has absolutely no idea of how to
calculate it. Instead of a numerical value, the user may characterize the
relative rotational inertia of the load as light, medium, or heavy. Of course,
the program has no way of knowing the actual inertia and it may lead to
selections that are over conservative. The best selection and the most accurate
performance prediction is obtained when the rotational inertia of the load is
known, or can be computed or measured.
Peak Torque to Static Torque in Depth: The peak torque is the maximum torque
that the driver must supply during start-up. The static torque is based on the
rotational speed and transmitted power specified by the user. The ratio of
these torques provides an indication of the required overload capacity required
of the driver. The selection program assumes that the driver has perfect speed
regulation regardless of the load. In reality, this is not the case, because
the length of the start time and the inertial load have a direct relationship
to the load the driver sees, and can overload the driver if the inertial load
is high and the start time low.
Work Capacity in Depth: This value is a measure of the facing life and is based on the
volume and composition of the facing material. Facings are offered in low,
standard, and high coefficients for most products with low providing the
longest life and then decreasing by approximately 40% each time
the coefficient is increased. If facing life is a major consideration in an
application, it is beneficial to use low coefficient facings although due to the
lower torque capability you may need to choose one product size larger. This
will initially be a more expensive proposition but the larger product will
provide more facing area, longer bearing life, and higher thermal capabilities
in addition to the low coefficient facings potentially doubling the products
life.
In
accordance with Nexen's established policy of constant product improvement, the
information contained in this product selection program is subject to change
without notice. Technical data listed on this website is based on the latest
information available and is also subject to change without notice. This
product selection program is to only be used as a reference and is not a
substitute for application and product knowledge. The data generated by this
program will only be as accurate as the input data. If you are unfamiliar with
the clutch and brake selection process please contact Nexen's technical support
group for assistance at (800) 843-7445 or service@nexengroup.com. In no event
shall Nexen be held liable for any consequential, indirect, incidental, or
special damages of any nature whatsoever, including without limitation, lost
profits arising from product misapplication based on the output of the
selection program.