The effect of a holding brake on motor torque requirements is usually minimal in a regular rotary or linear horizontal motion application and is therefore not necessarily recommended. However, the torque and power reduction can be quite dramatic in case of vertical linear applications. The main purpose of a holding brake is to relief the motor from maintaining the holding torque during standstill periods in a vertical linear motion application. The use of a holding brake can save energy, not only due to the application of the brake during standstill, but also due to smaller motor size requirements.

The downside of using a holding brake, however, is that it adds inertia to the motor load, therefore increasing torque and power requirements during acceleration and deceleration. Some criteria to use a holding brake could be:

• Brake inertia is much smaller than the load inertia

• Duty cycle includes significant standstill periods

The final decision to apply a holding brake should only be based on calculating the torque requirements peak/intermittent as well as RMS torque - of all possible motor/brake combinations and then select the motor according to the lowest torque requirements. This is, of course, a time consuming and tedious process, but yet again a good example where motor sizing programs are a great help.

 

 

Picture 1: Leadscrew Application

Picture 1 shows a linear leadscrew application with no holding brake and we assume the following parameters:

Leadscrew Inertia 0.00004158 in-lb-s²
Friction Torque at Leadscrew Input Shaft 0.5 in-lb
Leadscrew Mounting Angle                            0 degrees

The duty cycle is set to:

Total Duty Cycle Time 2 sec
Maximum Speed at Leadscrew Input Shaft 1000 rpm
Dwell Time                                                1 sec

Note that we (initially) chose a mounting angle of zero degrees to make this a linear horizontal application. We also added some constant torque which is represented by the friction torque at the leadscrews input shaft.

The resulting torque requirements in reference to the duty cycle will look as follows:

 

Picture 2: Velocity and Torque Graphs Horizontal Linear Application. 

Note: The above shown torque requirements do not include the motors inertia. The graphs were created by the VisualSizer-Professional motor sizing software.

Notice that the required torque during acceleration and deceleration remains very close to the constant torque represented by the friction torque of the leadscrew. During acceleration the added torque results from the leadscrews inertia, since torque equals inertia times acceleration. The same is true for the deceleration phase, however, in this case the torque is reduced.

The important part since we intend to add a brake eventually is the dwell time. As can be seen in picture 2 the total torque of the application will be zero during motor standstill, since this is a linear horizontal application, i.e. the motor does not need to maintain a holding torque. Ergo, in all consequence we do not need a holding brake. This picture will change quite a bit when we change the mounting angle to 90 degrees, i.e. by creating a vertical application.

 
Picture 3: Leadscrew Application with brake

First, however, lets add a holding brake to the current linear horizontal application and observe the effects of the holding torque. We are assuming a brake inertia of 0.004 in-lb-s² which is for educational purposes - significantly higher than the leadscrew inertia.

 
Picture 4: Effect of Holding Brake Inertia on Total Torque 

As picture 4 clearly demonstrates the effect of the holding brakes inertia on the total torque, especially the intermittent torque, is quite dramatic in this admittedly fabricated example. The point really is that the use of a holding brake will increase the motor torque requirements to a certain degree.

In the next step lets set the leadscrew mounting angle to 90 % and make this a vertical application, however, without applying the holding brake at zero speed.

 
Picture 5: Total Torque in a Vertical Application 

What happens here is that during acceleration the motor has to move the load upwards against gravity -, while, during deceleration, it has to move the load downwards with gravity. Common sense tells us that the torque during deceleration, where moving the load is supported by gravity, should be lower than during acceleration where load is moved against gravity. However, this is not the case. The exact details of torque calculation for vertical application will be explained in chapter 3.5.2 Vertical Applications.

In reference to picture 5 notice the current torque during dwell time, i.e. at standstill. The torque during dwell time is exactly 0.5 in-lb, i.e. the friction torque applied at the leadscrews input shaft. The motor needs to maintain this torque in order to keep the leadscrew in the current position and to compensate for gravity forces.

 
Picture 6: Total Torque with Holding Brake Applied

Notice yet again the torque during dwell time in picture 6, which is zero due to the applied holding brake.

The maximum (intermittent) torque required from the motor will be in both cases, with or without a holding brake applied, the same, namely 0.94 in-lb.

The main difference lies in the RMS torque, which is used to determine the motors rated torque, i.e. the torque it can maintain constantly.

The equation to calculate the RMS torque is as shown here:

Equation 1: RMS torque calculation 

Symbol	Description
Ta	Acceleration torque
Tc	Constant torque
Td	Deceleration torque
Th	Holding torque
ta	Acceleration time
tc	Time of constant speed
td	Deceleration time
th	Holding (dwell) time

 

Ta, Tc , Td and Th as shown in the equation represent absolute values. Deceleration torque is usually negative. The motor, however, has to provide at least that amount of torque to drive the mechanical setup during deceleration.

According to equation 1 the RMS torque, according to the total torque as shown in picture 4 (brake not applied at zero speed) would be 0.82 in-lb, while the RMS torque according to picture 5 (brake applied at zero speed) would be 0.77 in-lb. This demonstrates clearly that the use of a holding brake can be beneficial in terms of using smaller motors and thus requiring less energy.

However, as was stated at the beginning, it is important to calculate the torque requirements peak/intermittent as well as RMS torque - of all possible motor/brake combinations and then select the motor according to the lowest torque requirements.