MALPRACTICE
IN DESIGN AND MANUFACTURE: A CASE STUDY
Dr. Tesfaye Dama
Faculty of Technology, Addis Ababa University, Ethiopia
Published in the Journal of the ESME, Vol. II, No. 2, October 1999
Reprinted with ESME
permission by the African
Technology Forum
ABSTRACT
The paper very briefly presents the malpractice observed in design and manufacture of a drive unit of a machine, claimed to be of a foreign origin. Sufficient evidences, obtained through visual inspections of the unit are presented here to expose the poor and unconventional design, manufacture and assembling of the drive unit.
Introduction
Observations
and Analysis
Gearbox
Lubrication
Gears
Bearings
Bearing Housing
Gearbox Cover
Pulleys
Remarks on Imported Machines
Photographs of the Gearbox
Conclusion
Technological development is a prerequisite for the industrial development of a country, which in turn is a key factor for economic development. One of the modes of technological development is through technology transfer. This can be from inventor to exploiter, from one enterprise to another, from one country to another. If we single out the last mode of the technology transfer, it can be categorized as follows:
Transfer on a contractual basis, e.g., licensing;
Training;
Import of technically advanced products; and
Information retrieved from patent documents.
So we have several of the options for transferring technology from advanced countries to countries like ours. If we dwell on the import of technically advanced products, this introduces the current systems developed, materials being developed, etc. and allows or encourages for further development of machines on the basis of these imported technologies. Furthermore, it can trigger the knowledge we already have and can open up a horizon for academic exercise in design and manufacturing. Following such procedures, technological development of a country can be realized effectively. The products developed and manufactured on the basis of such procedures will definitely work and be reliable since there is an element of design knowledge incorporated in the process.
In some cases, however, imported products from developed countries may not meet the development level (design, quality of fabrication and assembling) demanded and expected. There are several reported cases that involve such malpractice and supports this statement. The objective of this article is to present a case of a machine (in particular the speed reduction unit) investigated by the author and claimed of foreign origin.
OBSERVATIONS AND ANALYSIS
In the following sections the observation and analysis made on the elements of the drive unit are presented in some detail.
The gearbox mounted on this machine attains a considerable speed reduction to drive a screw conveyor. The speed reduction goes from 1450 rpm on the electric motor side to about 28 rpm on the screw conveyor. Together with the electric motor they form the drive unit of the machine. In addition to this, the crucial moving parts of the whole machine are contained in this unit. As this is the most sensitive part of the machine, it must be designed very carefully so that all the rules of conventional gearbox design are strictly followed.
A glance at the gearbox of this machine shown in Picture 1 suggests that the gearbox is a modification, and not the original one.
The design and assembly of this particular gearbox are completely out of phase with that of the conventional mechanical design practice. These are discussed in the following sections. Sectional views of this gearbox are shown in Fig. l.
Fig. 1: Sectional views of the gearbox of the plodder with bearings welded to the housing and gears welded to the shaft.
The life support of gearboxes is lubrication. Lubrication requires the use of correct oil, the maintaining of safe oil level with the help of oil level gauges, convenient oil refilling and drain systems and prevention of dust and other particles from entering into the oil by using correct oil seals and plugs.
The gearbox observed does not have an oil level gauge. The refilling hole is open and it allows dust and other particles to mix with the oil, thus deteriorating the lubrication quality of the oil. The presence of an oil level gauge allows the operator not to overfill or under fill the gearbox with oil. The refilling hole is shown in Picture 2. At least it could have been threaded to cover it with a bolt to prevent oil loss by splashing and avoid entry of dirt.
On the same picture, the oil seal used for the gearbox casing is visible. The material used is an inner tube of a tyre, which is not a proper material for oil sealing, as it disintegrates when in contact with oil products.
All the gears used in the gearbox are helical type. In addition to transmitting torque, helical gears also transmit axial forces and for this reason the gear must be rigidly mounted on the shaft. Large gears can be mounted on by keys and press-fitted joints or usually with spacers (sleeves) to eliminate axial movement. On the other hand, small gears are usually made integral with the shaft. Mounting of gears on shafts by welding is however, most unconventional and unacceptable practice as the joining process results in:
Deformation of gear teeth and the shaft due to heating;
Unbalance of the rotating parts which in turn are sources of vibration; and
Disallows disassembling for maintenance.
A total number of five gears of this gearbox are mounted by welding. Three of these gears are shown in close range in Picture 3.
The sixth gear is mounted on the shaft by key-like pins. The mounting of this gear is shown in Pictures 4 and 5. The gear is welded to a split hub and key-like pins to secure it to the shaft for torque transmission. The purpose of inserting the split hub must have been to secure the available gear wheel of larger internal diameter on a smaller diameter shaft. In addition, there is too much non-uniformity of clearance between the hub and pins and the mountings of the pins on opposite sides are not radial as shown in Picture 4.
All shafts including the bearings are not properly secured axially on the gearbox housing, as shown in Fig. 1. This results in a direct contact of the bearing and the shaft face with the housing which will have a serious operational effect and also a damaging effect on the gearbox cover. In this case, a sleeve was used to eliminate the axial movement of the gear. However, it was noted that there was a very considerable clearance between the sleeve and shaft that could result in eccentric loading on the ball bearings.
These mountings of the gears on the shafts clearly indicate that they were manufactured by one who has no knowledge of the rudiments of machine design. Such kind of mounting would result in serious malfunctioning of the drive unit and its lifetime will be considerably reduced.
All bearings employed in the gearbox are deep groove ball bearings. These particular types of ball bearings have high degree of conformity between balls and grooves and they give no allowance for misalignment of shafts. Looking at the nature of the forces involved in this gearbox, bearings with high axial load capacity (in addition to the radial load) and self-aligning capability must have been used.
When bearings are mounted on shafts and housings, they must satisfy the tolerance requirement for the intended purpose. For example, tolerances for gearboxes of this type are k6 and M6. The k6 designation specifies that the shaft diameter is greater than the inside diameter of the bearing. This implies that the fitting between shaft and inner race of bearing is one of interference fit which could require considerable force on the inner race or heating the bearing in a bath of oil to insert the bearing on the shaft. The M6 designation specifies that the outer diameter of bearing is greater than the diameter of the housing. Again the bearing must be inserted into the housing by applying some force on the outer race of the bearing. One of the bearings is shown in Picture 6.
Measurements on the six bearings inside diameters showed to be 30 mm while the diameters of the shafts that fit into these bearings ranged from 28.65 mm to 29.95 mm. The seventh bearing had an inside diameter of 40 mm while the shaft was 39.95 in diameter.
Contrary to the tolerance requirement, there is quite a wide clearance between the shafts and the bearings. This has an adverse effect on the operation of the gearbox, which could result in:
Relative motion (slipping between the shaft and inner race of bearing) which will wear out the shaft, the degree of the wear depending on the speed of rotation of the shaft; and
A misalignment of shafts easily damages the bearings. The misalignment will be greater if the shaft also wears out and in the extreme case there may be no meshing of the gear teeth at all as shown in Picture 7, in this particular case.
Operation of the gearbox under such condition will drastically reduce its life. Such kind of an assembly can only be manufactured by someone who does not have any idea of the elements of machine design.
A great care is necessary while manufacturing, to ensure the alignment of the bearing housings, while simultaneously maintaining the interference fit between the outer race of the bearing and the housing. This interference fit will prohibit the outer race from rotating relative to the bearing housing, so that, it will not wear out the housing. Wear on the housing causes misalignment of shafts and slippage whose effects have been seen earlier. In this particular gearbox, all the bearing housings (cylindrical rings) were simply welded on the side covers of the gearbox. One of them is shown in Picture 8.
With such kind of manufacturing there is no way of ensuring the alignment of shafts. In one particular case, the bearing could be pulled out from the housing by a touch of a finger indicating a fitting with a clearance between the outer race and the housing. This is absolutely not allowed. A misaligned shaft reduces the life of the bearings and over stresses some sections of the gear teeth due to imperfect meshing.
Another striking fact is the absence of a provision for the removal of the bearing from the housing incase of replacing it due to damage. Unless the fitting between the outer race and the housing is that of clearance fit there is no possibility of removing a damaged bearing. For interference fit one need to apply a force for pulling out a bearing. In this case, there is no spot where the withdrawal force can be applied as shown in Fig. 1. However, provision for disassembling of the bearings was essential.
The pinion shaft is driven by an electric motor through a belt. This shaft protrudes from the gearbox so that it can be coupled to the motor. The hole made by this protrusion on the gearbox must not allow any object such as dust and other particles to pass through, so that the lubricant will not deteriorate and flow to the outside. For this reason, the hole is made perfectly circular and correct lubricant seals are used.
The manner in which the hole was punctured for the shaft to pass through is shown in Picture 8. There is no way of sealing the bearing which is right behind the cover and also the lubricant will deteriorate as it is exposed to dirt, dust, moisture, etc. Lubricant can also flow out through this rugged hole. Such kind of extremely poor manufacturing cannot be expected from any manufacturer.
Part of the driven shaft pulley is shown in Picture 9. This pulley is made of aluminum. As shown in the picture a sleeve made of stronger material is used to transmit the torque. The sleeve is cut all the way for a key way. What will stop the sleeve from rotating? There is no way that the interference fit of the sleeve can be maintained!
REMARKS ON IMPORTED MACHINES
When a client receives imported or local produced machines, the first thing that he is expected to do is to test the machine if it meets the specification set by him. The whole process of testing the machine is called commissioning. Commissioning includes conducting rigorous tests on the machine which begins with simple visual inspections to ascertain if correct procedures have been followed in the design and manufacture of the machine. This is followed by series of operational tests to check if it meets the output required such as final speed, capacity, quality of product or other parameters. It is only after the success in these tests that the machine is accepted by the client.
If the machine does not pass these tests, the burden will become heavy, especially if it is an imported machine. This may entail a replacement of the machine which could include shipping back the unsuccessful machine. And this becomes a very expensive affair for the supplier.
PHOTOGRAPHS OF THE GEARBOX
Picture 1: Gearbox with the rear cover partially painted. Also shown is the electric motor.
Picture 2: Cover of the gearbox removed. a) oil seal b) refill hole
Picture 3: Gears welded to shaft.
Picture 4: The gear is welded to a split hub and the hub is held by two key like pins to the shaft. Observe the wide clearance.
Picture 5: Poor orientation of the pins. At least they should have been radially oriented.
Picture 6: Welded bearing housing. There is no spot to apply the withdrawal force.
Picture 7: The misalignment created by big clearances can be so great that the gears may not mesh.
Picture 8: A rugged hole punctured at the rear cover of the gearbox (also shown in Picture 1).
Picture 9: The sleeve can not generate the required force from a shrink fit.
The drive unit is the most important component of the machine. When considering the manner in which it was manufactured, it may not even be worth calling it a drive unit. No conventional procedures were followed for its design, manufacture and assembly. This puts it into the category of a very highly unreliable and very cheap unit. It will not even pass the first test of equipment commissioning. Therefore, from the presented case, the message of the paper to practicing engineers is to advise to strictly check for the possibility of malpractice in design and manufacturing whether the machines are locally produced or imported. Finally, though we have to encourage reverse engineering of machines locally, we have to adhere to the proper design and manufacturing for safety, life and satisfactory economic return.
ACKNOWLEDGEMENT
This is a part of a report submitted by Ato Gizachew Shiferaw, Dr. Demiss Alemu, and Dr. Tesfaye Dama dealing with investigation of a machine. It was rewritten to give it the form of a paper for a journal.
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