Simple solutions save maintenance cost - a case study
“Condition Monitoring System”, short CMS, is more than just a buzzword in the maintenance business. It stands for continuous condition monitoring. The goal is to minimise downtime. If arising damage is predicted, maintenance can be optimally planned. A machine part is exchanged exactly when the wear margin is almost fully exhausted. Only in this way the efficiency of the system can be maximised. Experts agree: 90 to 95 percent of all failures are preceded by measurable damage behaviour. But the difficulty lies in detecting the exact moment.
CMS monitors minimal changes in the conditions of system parts, for example via vibration measurement on rolling element bearings. Due to the continuously measured data a comparison with predefined limit values is carried out. Any deviation is automatically signalled to the monitoring unit and actions can be planned. The jam and confectionary manufacturer Zentis in Germany, is one of the first to use such condition monitoring systems. In the central cooling system a huge cooling fan with a diameter of nearly three and a half metres carries the heat away. The unit including motor and gear is mounted outside on a cooling tower. The rotor blades are exposed to extremely strong forces when they rush with 2,000 revolutions per minute through the air. Even tiny deposits and wear cause unbalance. An enormous stress for bearing and gear.
A fault would have fatal consequences: "Especially in the food industry we depend on specified temperatures. If the cooling fan failed the whole cooling system would break down and with this also the three-shift production for about a week", says Romeo Odak, responsible for technical equipment and installation at Zentis in . The financial impact is obvious. And due to the size of the plant the system cannot be designed redundantly.
In the past the cooling fan was regularly inspected with a lot of manpower and material input involved. Bearing damage is mostly noticed acoustically, “five people and five opinions on what kind of irregular sounds are perceived”, says Odak. 50 percent of the decisions whether something should be replaced were taken objectively, 50 percent subjectively. In case of doubt a piece was replaced rather than to risk a failure. However one barely reached the wear limit. It was “learning by doing”, soon preventive maintenance became routine which meant regular replacing irrespective of the real stage of wear.
Continuous Monitoring Via CMS
At the heart of this system is a small and compact sensor for rolling element bearing diagnosis. It permanently monitors the acoustic emission and calculates thereof the frequency spectrum from this. This allows to reliably evaluate the condition of the bearing.
The sensor was directly mounted at the gear of the fan. After quick parameter setting and teach-in the unit was ready for use. The sensor displays the condition of the bearing via a 9-digit row of LEDs in traffic light colours. “We were surprised about how precisely the sensor indicated the increasing wear of the gear over months”, says Odak. “When the indicator of the unit had reached the yellow zone we could calmly plan the replacement of the system.” When the system eventually indicated the “red phase” the manufacturer was asked to replace the cooling fan. “We were able to calculate exactly when the replacement should take place and worked in advance in the production. In this way the replacement did not cause any production loss.”
“We were keen to know how far the wear margin of the gear was consistent with the indication of the sensor. That is why the old gear was transported with a flat bed truck to the manufacturer where it was disassembled. The examination showed: around 90 to 95 percent of the wear margin was used - exactly the value that also the sensor had indicated.
By now Zentis uses more than 40 sensor units. “Also for other systems the bearing conditions were exactly indicated. This allowed us to replace the worn out components at their maximum life-span without ever having to worry about a standstill”, confirms Odak. “Correct maintenance using CMS strategies allows a reduction of 25 to 40 percent in maintenance costs.”
The sensor is a vibration sensor with integrated rolling element bearing and machine diagnosis based on frequency analysis. It allows permanent monitoring of small machines and components while featuring the same diagnostic quality as expensive systems. A small unit typically monitors up to two different rolling element bearings. A larger unit monitors up to twenty different frequencies.
Damage to rolling element bearings is detected as early as it arises. The bearing monitor sets the switching output 1 and indicates damage progress with yellow LEDs. The operator can take preventive action. Progressive damage to rolling element bearings is signalled via the second switching output 2 and is indicated via the bearing monitor's red LEDs. The operator should react at once. The basic parameters are set on a PC. Data such as type of bearing, rotational speed and input / output function are transmitted to the sensor via the RS 232 interface. The reference conditions of the machine are taught after mounting by pressing a pushbutton on the sensor.
- Bipin Jirge