The test case fan is equipped with a 2.2 kW electric motor, which is controlled via variable-frequency drive. The motor nominal speed is 1440 rpm. The fan is used in air handling units and typically equipped with rubber elements. In this article InfiniSpring®-springs are dimensioned below the fan.

Fan weight

The fan weight was first measured to determine spring selection. Measurement results were taken below the existing rubber elements and measurement results are shown in Figure 1.

Figure 1. Weight measurement results for the fan.

Spring selection

The fan originally had four springs and four InfiniSpring®-springs were chosen to be used in the fan. Spring selection can be made with following steps:

• Calculating total mass: 17.0 kg + 17.0 kg + 23.0 kg + 23.0 kg = 80.0 kg
• Deciding required number of springs: 4
• Calculation load for one individual spring: 80 kg / 4 = 20 kg
• Choosing optimal spring based on the individual spring load: 3HS20 spring with nominal load range 15-20 kg according to Table 1 below (table is taken from the datasheet found on the Product page)

Table 1. Vertical nominal loads of a single spring from the datasheet.

Spring locations

Final step is to dimension springs symmetrically around the center of gravity in the fan axial direction. Fan motor axial direction is referred as X-direction in Figure 2. The zero-point location is also shown in Figure 2.

Figure 2. Fan equipped with original rubber elements.

First the centre of mass x-coordinate is calculated. After this the location for the motor end spring is chosen. In this case 65 mm from the end of the 584 mm long attachment aluminum bar is used, Figure 3. Spring locations from the zero-point location are then calculated as follows.

Calculation steps

• Calculating centre of mass x-coordinate:

• Deciding motor end spring location: 65 mm
• Calculating spring locations:

Figure 3. Spring locations

Fan with new springs

Fan with 3HS20 InfiniSpring® is shown below in Figure 4. Individual InfiniSpring® is shown in Figure 5. Optimal springs were selected for the fan and the fan is ready for operation. Safe operational speed ranges from 755 rpm upwards. Minimum operational speeds can be found from the Data Sheet and also shown in Table 1 above. Please see also a video below introducing the fan and InfiniSpring®-springs.

Figure 4. Fan equipped with 3HS20 InfiniSpring®

The post How to select InfiniSpring® for a fan? appeared first on InfiniSpring.

Test case fan is equipped with 3HS20 InfiniSpring®-springs and with a 2.2 kW electric motor, which is controlled via variable-frequency drive. Motor nominal speed is 1440 rpm. This article focuses on testing the fan with big unbalance to see which rigid body modes are harmful for the fan installation. A fan with a big impeller unbalance mass (extra bolt) is shown in Figure 1 below.

Figure 1. Fan impeller with big unbalance mass.

Completed Test

The fan was tested with a big unbalance mass in different rigid body mode (flexible installation natural mode) resonances by setting the motor speed to measure natural mode frequency of the flexible installation. In practice the fan speed was set first to fan longitudinal mode frequency 4.5 Hz (270 rpm) and fan vibration levels were checked. After this, the speed was raised to fan transversal mode frequency 4.75 Hz (285 rpm) and again fan vibration levels were checked. This was continued all the way up to the last natural mode. In total there are always six degrees of motion (3 translation degrees of freedom and 3 rotating degrees). Below are shown six tested rigid body natural modes and their frequencies.

Test with big unbalance

The test revealed that only transversal and vertical natural modes are excited with big unbalance. Other rigid body natural modes are not excited. Below is a video showing the fan is speeded up from 0 Hz to vertical mode 9.5 Hz (570 rpm) with a very big impeller unbalance. The fan vertical rigid body mode is excited heavily in the vertical direction.

With InfiniSpring® vertical rigid body mode always has a higher natural frequency compared to transversal rigid body mode. Due to this, the vertical rigid body mode defines the minimum operating frequency for the fan. For the fan that was tested, the vertical rigid body mode is 9.5 Hz (570 rpm) and transversal rigid body mode is 4.75 Hz (285 rpm).

Conclusions

Based on the test transversal and vertical rigid body modes for the fan are excited with big unbalance. They are therefore the most harmful rigid body modes. Other rigid body modes are not excited.

Vertical rigid body mode frequency is always higher than transversal mode frequency. Due to this, the vertical rigid body mode frequency is used for dimensioning InfiniSpring®-springs for a fan, and it also defines the fan minimum operating frequency.

Datasheet values can be used to define minimum operating frequency. The datasheet defines the minimum rotational speed, e.g. 3HS20 InfiniSpring® is for rotational speeds above 755 rpm (12.6 Hz). The rpm value can be found from the datasheet.

P.S. In this case, the fan can be operated safely in a speed range from 755 rpm up to the speed allowed by the fan or motor.

P.P.S. With the balanced fan, none of the rigid body modes were excited and it was safe to operate the fan from 0 rpm upwards.

The post Which frequencies are harmful for fan installations and what is fan’s minimum operating frequency? appeared first on InfiniSpring.

The test case fan is equipped with 3HS20 InfiniSpring®-springs and a 2.2 kW electric motor, which is controlled via variable-frequency drive. The motor nominal speed is 1440 rpm. The fan is used in air handling units and typically equipped with rubber elements. This article focuses on validating InfiniSpring®-spring selection based on the fan weight data. Fan with 3HS20 InfiniSpring® is shown in Figure 1 below.

Datasheet Data for 3HS20 Spring

According to technical datasheet 3HS20 spring is for nominal load ranges between 15-20 kg and for rotational speeds above 755 rpm. Maximum vertical natural frequency is less than 11.7 Hz, Table 1 below.

Table 1. Maximum vertical natural frequencies when using nominal load range.

Natural Frequency Validations

The vertical natural frequency was measured to validate the limit value 11.7 Hz. The measured vertical natural frequency result is shown below in Table 2 together with the datasheet limit value.

Table 2. Measured vertical natural frequency.

In order to comply with the data sheet value, the measured value needs to be below the datasheet frequency 11.7 Hz for vertical natural mode. Based on the 9.5 Hz measurement result we are clearly on a safe side. It is to be noted that the test case fan 3HS20 InfiniSpring® is for a load range of 15-20 kg and the spring load is 20 kg. This means that 15 kg spring load would give vertical natural frequency of approximately 10.5 Hz and the result would be safe with lower load.

Conclusions

Based on the measurement the vertical mode frequency for the test case fan 3HS20 InfiniSpring® is from 9.5 Hz to 10.5 Hz and vertical mode frequency is always safely below the datasheet value 11.7 Hz. Weight data can be used very safely for the spring selection.

Fan and InfiniSpring®-springs introduced below.

The post How safe is the spring selection based only on the weight data of a fan? appeared first on InfiniSpring.