MICRO FOG MACHINE LUBRICATION
Reproduced in extract form from 'Machinery Lloyd' and 'Electrical Engineering' (Vol. 37, No. 8 - 10th April 1965) With kind permission of the publishers – The Certificated Engineer March 1966.
The first micro fog lubricator produced by C. A. Norgren Co. USA, was in 1950, and it was thought, at that time, that such a system of lubrication would be applicable only to small high-speed precision devices.
Before the advent of the micro fog lubricator oil fog lubrication had been used for lubricating not only air powered devices but also bearings of precision grinding equipment. However, due to further developments of this type of equipment the oil fog lubricator concept was no longer applicable because even a minute drop of oil entering bearings at the speeds at which the machine were operating caused chatter in the grinding spindle, and this what not acceptable. This led to the development of a superior lubricator, one which would produce a more uniformly divided oil fog. The result was the micro fog lubricator. Up to this point no heavy equipment had ever been lubricated by means of an oil fog.
While early developments of aerosol lubrication were concerned mainly with its application to machine tools it has not been used in this field as extensively a originally envisaged, where as the steel and aluminium rolling industries which, in the early stages, were somewhat doubtful of its efficiency now accept it. It is interesting to note that by May, 1964, the total number of mill stand equipped with an aerosol lubrication tern to the roll neck bearing was 44. This figure refers to new rolling mills (hot and cold) either built or n tailed in the UK.
Machines to which aerosol lubrication has been applied include paper making and printing machinery, packaging machines, powder grinding mill, centrifuges, machine tool grinding spindles. drilling machines, horizontal borer broaching, vertical milling, and gear hobbing equipment.
In the machine tool field aerosol lubrication has done way with some awkward lubrication problems, especially those associated with equipment incorporating swivelling heads, and splash, or pump, lubrication systems to gearboxes. When using these types of system precautions have to be taken to avoid spillage from the gearbox, and to see that gears are continuing to dip into the gearbox when they are part of a swivelling unit. Another interesting point is that bearings lubricated by aerosol techniques run cooler. This is important because it enables the accuracy of a machine to be more easily maintained.
In the automotive industry aerosol lubrication systems are found on some transfer machines, special multi-head tapping machines. valve grinding equipment and crankshaft drilling machines.
Apart from the rolling mill applications already described, aerosol lubrication systems are employed in strip straightening bridles, rolling mill screw-down gears, cold tube reducers slab millers, strip min loopers and guides. bar drawing machines, section bending machines. and Cooling machines.
Reason for Employing Aerosol Lubrication System
While many reasons are given for employing an aerosol lubrication system they can be grouped roughly under the following headings:
Bad design: This seems to occur much more frequently than it should in that a machine is designed without sufficient consideration being given to its method of lubrication. Not infrequently an aerosol lubrication system prove to be the only practical automatic lubrication system that can be installed.
Modernisation: Because many machines are being modernised it is often found that the existing lubrication system is not good enough. In many instances the system is operated manually, and this alone precludes its use with the higher speeds now in force.
Cleaner operation: Due to ineffective sealing methods some of the older machines tended to have oil leaks when using either manual or pump oil systems. In these cases aerosol lubrication system have been fitted rather than undertake the more difficult ta k of installing an up-to-date sealing system.
Heat: Problems have occurred for many years on bearings exposed to high ambient temperatures, particularly in the steel industry where such temperatures have been overcome.
Increased speeds: Increasing the speeds of hafts and gears has led to a greater interest in micro fog lubrication since it avoids the problems caused by churning of the lubricant.
Deliberate design: Machines are now being designed from their inception to employ micro fog lubrication system. In many cases where an aerosol lubrication system has been planned from the beginning it is found to be simpler to apply than any alternative system.
Details of the Micro Fog Cabinet
A micro fog cabinet which has been developed is the outcome of requests for a unit which looks like a piece of the machine tool, to which it is to be integrated. The cabinets now in use vary in design since they house different pieces of equipment dependent on the requirements of the customer.
The essential element in the micro fog lubrication system is compressed air. Since it is necessary to have clean compressed air a filter is needed, and because the lubricator works at a lower pressure than the airline, a pressure regulator is required. At the point of lubrication reclassitiers are needed to reclassify the finely divided fog. Other pieces of equipment that are sometimes included in the cabinet are: a pressure gauge, liquid level control, and solenoid valve pressure switch.
In some cases cabinets include two pressure switches, one to give a signal or to initiate some action, even to shutting down the machine should the air pressure fail, and the other to because a as protective device for the lubricator reservoir. If a solenoid valve is included in such a set-up then it could be used to start and stop the lubricator simultaneously with the machine. A liquid level control can be used to sound an alarm should the oil level run too low.
Air and Oil Heating
Another type of unit combines air and oil heating. The concept of air and oil heating first carne into prominence three years ago; the principal object was to enable users of aerosol lubrication to use higher viscosity lubricants. Without the benefit of heating devices the maximum viscosity of the lubricant was at 1000 SSU (about 850 sec Redwood). By employing air and oil heating, and keeping the oil temperature in the vicinity of 85° F to 90° F and running the air temperature up to 120° F, it became possible to create and convey a fog using lubricants having a viscosity as high as 25 000 SSU (about 21 000 sec Redwood).
While some industries insist on using viscous lubricants there is a general feeling that they are not really necessary. Today with the more advanced technology of lubrication and the EP additives and oxidation inhibitors there is less justification for using these extremely viscous lubricating oils.
There are many applications for oil heating alone without air heating but, 0 far as i known, there are no applications where an air-heater would be used by itself. There are three reasons for employing an oil heater. which are, in order of importance:
(A) To ensure constancy of oil output from the aerosol lubrication system, despite fluctuations in ambient temperature. This would not, normally, require any form of air heating.
(B) To raise the oil temperature by a limited amount to between 80 and 90° F and maintain it within those limits. It then becomes possible to secure an acceptable aerosol output from oils that have negligible output at temperatures in the region of 60°F. There are also certain borderline oils which have a limited useful aerosol output at temperatures in the region of 60°F but which have excellent aerosol output at temperatures between 80 and 90° F. Here again there does not appear to be any need for air-heating.
(C) It is possible to create micro fog from very high viscosity oils if the temperature of these oils is raised to figures in excess of 140° F. The e application certainly requires the use of an air heater as well.
The first application mentioned above is an extremely important one in heavy industry.
The second application can also be helpful with certain oil used in heavy industry especially the steel industry, and al 0 for oil required to lubricate heavily loaded low speed gearing.
The needs for very high viscosity oils is unusual: however, if these oils are called for then an air heater is definitely essential as well. Also, there may be condition where air heating is essential; for example, under Arc condition.
Problems Associated with Lubricating Oils
Initially problems are due to lack of knowledge of the behaviour of various oils when broken down into an aerosol-coupled with the fact that often the system was required to use an oil that was already in use. The latter point was sometimes necessary when breaking new ground so that only the method of supplying the lubricant was changed, and then having proved the system it was possible to introduce alternative grades of oil.
Early applications showed that no two oils fog alike and invariably the major problem was one of free fog, i.e. oil which, having been broken down into an aerosol, did not easily change back into a wetting oil but escaped into the atmosphere like so much cigarette smoke. These early application showed the importance of correct oil selection, based not only on the oil's ability to do the job but its ability to be got to the point of application.
One of the first questions to be answered was how much or little oil was necessary to provide satisfactory lubrication under all sorts of conditions. To ascertain the answer to these problems a series of tests were undertaken.
The test consisted of placing a measured quantity of oil in the lubricator and running the system for two hours, measuring (1) the oil collected from the reclassitiers, (2) the oil left in the lubricator and (3) that which had fallen out of suspension and collected in the manifold catch trap. The matter of free fog was resolved on the company's experience of what would, or would not, be tolerated.
The oil's free fog was rated by visual means. This was then classified into one of six groups varying between grade 0 for no free fog, grade 2 for free fog visible but not objectionable, to grade 5 for too much and not acceptable. Temperatures were recorded and there were kept within 65 to 75° F.
These tests also showed that the amount of oil output can vary with the degree of aeration of the oil. It was noted that some oils deliver next to nothing during the first 10 or so minutes of operating and then build up a steady output. Other oils start straight away and maintain a constant output. Therefore during the tests he amount of oil delivered at 15 min interval was noted.
If a particular oil fails the first test as regards output there are two further tests which can be applied. An Increased air pre sure can be put into the lubricator (but not into the manifold) to aid the breakdown of oil into fog. Secondly, the oil can be heated; this allows the fog delivery rate to increase. Some initial test show that an oil tested and found not suitable at standard test pressures becomes a very suitable oil if first heated to between 90 and 100° F.
If too much free fog is evident this can sometimes be reduced by increasing the manifold pressure from the recommended 12 to 16 in WG, up to 24 to 30 in WG or in extreme cases to as high as 40 in WG , but this brings in another problem-that of manifold deposition. Higher manifold pressures and the consequent increased velocities cause more oil to fall out of the airborne u pension. Of all the problems connected with oils and aerosol lubrication. free fog is the biggest one, for it represents a loss of efficiency in the system. In connection with this problem tests showed that the addition of a small quantity of 10 W-30 multi-grade oil had the ability to reduce the free fog output of a high rate fogging oil furthermore the rate of output of the original oil is not materially affected. However, this mixing of oils has certain disadvantages and it is not recommend since there is a tendency for large deposits of oil to accumulated in the manifold.
The variation of oil delivered and oil deposited in the manifold can give rise to a further problem banned on the complexity of the pipework in the system. A small simple system with relatively few outlet points and a downward run of piping could use a low output oil, because any which deposited itself in the pipe would run down to the lubrication points. On the other hand a large multi unit complex system piped with rising manifolds must use an oil with good oil delivery qualities and low manifold deposits in order to prevent a build up of fluid dams in the distribution pipework.
Some other problems concerned with the delivery of the lubricants such as whether or not continual aeration of an oil would undermine its qualities are still being studied by the oil companies.
Possible Limitations in the Use of Aerosol
The application of an aerosol lubrication system to a machine may be limited because of the complexity of piping that is sometimes required. This applies particularly to equipment which i being converted. In these cases the number of points requiring lubrication is often large, and there is little room for mounting the necessary piping and lubricating equipment. Consequently the existing scheme of lubrication might well be the best.
Another interesting problem arises when a user decides to run a bearing at a higher load or at a higher speed. He usually wishes to know whether aerosol lubrication an help him attain these ends. There is obviously me limit to the amount of overload that can be put on a bearing by helping it along with aerosol lubrication.
Some heavily loaded plain bearings lubricated by an aerosol system are a source of worry because there is a feeling that there must come a point in the heavily loaded bearing where pressures will break down the film put on by the aerosol system of lubrication, and there might be some failure of the bearing. This problem also arises with regard to heavily loaded gears where the contact pressure could be too high for this type of lubrication. It also applies to tilting pad thrust bearings which have not, at the moment, been successfully lubricated with an aerosol.
It is possible that a shaft, a gear or a chain running at a very high speed might limit the application of an aerosol lubrication system because there is a tendency for a barrier of air to be created by the motion of the rotating parts, which prevents oil from reaching the bearing. In such cases a pre sure jet reclassifier, which is a source of additional air, helps to 'punch' oil through the air barrier, but there seems to be a limit to the speed at which that type of reclassifier can be effective.
While problems have arisen when using aero lubrication for adjustable and preloaded rolling contact bearings such as are used in machine tool headstocks, there is every confidence that a satisfactory lubrication system will be forthcoming for these types of bearing. Other limiting factors may be economic. For example; while aerosol lubrication could be successfully applied to the bearings of in electric motor-which usually has sealed-for-life bearings-there would be no economic justification for changing over the system.
Conclusions
Since there are so many variables that can upset the introduction of a new lubrication system it is difficult to lay down hard and fast rules and say that a particular mechanism or a machine can be satisfactorily lubricated by means of an aerosol system. Fortunately the application of an aerosol system to a particular piece of equipment has often led to the successful solution of problems in some other fields of industry.
While aerosol lubrication systems are being applied in increasing numbers throughout industry it must not be taken as the cure all for otherwise faulty systems. There are many applications for this type of lubrication that have not been tried out, and there are many questions to which there are no answers at the moment.