Application of optical streak sensors in air+oil lubrication systems

Sensors can be mounted in retrofit procedures in order to verify the working of the air+oil mixers that have been previously installed. In this case they are mounted directly at mixer outlet.
Please consider that in this case the sensor controls a flow that is never steady because the lubricant dosing, in single-line systems, takes place cyclically at timed pulses.

So we have to distinguish between a dynamic type control (signal at every lubrication cycle) or static control (signal nearly uninterrupted).
We have the following options:

1. Dynamic control: with sensor type IFX-S08 mounted inside the mixer (MVF-AX)
2. Dynamic control: when the sensor (type IFX-C04 or C06) is outside the mixer, but at a distance of max. 10 cm.
3. Static control: when the sensor (type IFX-C04 or C06) is outside the mixer and it is positioned to a distance of more than 1 mt.

In cases 1 and 2 the sensor will respond changing the status (LED green and then red) at every lubrication cycle, advising that the lubricant dosing took place properly. In case 3, the signal (green LED) is always on, provided that the lubricant flow rate (the oil streak) is enough and uninterrupted: for example, with a sensor positioned at 3 mt. from the mixer and a lubricant volume of at least 300 mm3/h, with an air flow not higher than 50 Nl/min.
A lower quantity of lubricant volume, or a higher air flow rate, can cause an unstable signal owing to a too limited and irregular flow for the optics of the sensor, but however valid for the needs of the lubrication point. This must be taken into consideration reasonably in the signal control management.
On the basis of the experience consolidated in the practical field, it is preferable to recommend a dynamic control, since this type of control is independent from the dosed volume and guarantees the control of every single cycle (“miss shot”).

The static control is used as redundant control, as guarantee of the lubricant presence in the final part of the pipes (see also video clip “redundant sensor”)

A monitoring system with dual oil streak detection sensors (optical sensor IFX) is considered as redundant. Redundancy is the duplication of components able to perform the same function, with the intention of increasing the system’s reliability. In this way a faulty element does not cause the system’s failure because the particular function can be performed by another element. A “failure-safe” system does not imply that failure is impossible/improbable, but rather that the system's design prevents or mitigates unsafe consequences in the event of a failure.

MWM has developed a new lubrication system with dual oil streak optical sensors, able to deliver extra safety to any air-oil lubrication process for spindle bearings.

A first optical flow sensor, IFX-S08, mounted inside the MWM’s patented air-oil mixer MVF-A, performs in-line oil streak dynamic monitoring. One additional optical flow sensor IFX-C, mounted external to the pipe in a position away from the mixer but close  to the spindle, performs static monitoring. This external sensor gives the operator full visibility over the continuity of the oil flow, as it is detected near the lubrication point. In case of a failure, the sensor gives an alarm signal and may stop the machine, to avoid any damage.

The meanings for dynamic and static monitoring are respectively as follows:

• dynamic monitoring:  during the lubrication cycle, when the pump feeds the lubricant to the air-oil mixer, the sensor detects the presence of lubricant (green LED). Once the first “wave” of oil is passed by, the air flow cleans the air duct and therefore the oil streak sensor does not detect any oil passage (red LED)
• static monitoring:  in normal operating conditions, a continuous streak of oil is transported by the air flow towards the lubrication point (spindle). The sensor mounted on the pipe, detects the presence of lubricant in a continuous mode (green LED). When this streak is no longer continue, the led turns red.

It is recommended to mount the external sensor IFX-C at not less than 1 m distance before the spindle, so that the residual pipe length may act as an oil safety reservoir. In case the external sensor detects no oil streak, there will be still sufficient time (10 minutes as a minimum) before the oil reservoir comes to its end. Therefore it will be possible to make a prompt intervention, avoiding the risk of leaving the spindle in operation without lubricant.
Instead, placing the sensor right before the spindle would be a critical choice: in case of failure there will be no residual pipe length which can be used as an oil reservoir for a short period of time. However, the redundant system will generate a (double) alarm signal and will immediately stop the machine.

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