Threads in SPIN-ON filters
Nowadays the most popular construction of filters applied for purifying all kinds of liquids is a can- type filter, commonly called SPIN-ON. Among lot of external differences which we can observe while looking at modernly produced filters, different thread size is the most noticeable one. The size of thread in the cover and particulars of the internal construction determine application of this very filter for usage in certain automobiles or installations.
Threads in SPIN-ON filter covers are cut in accordance with mandatory international standards concerning rolled screw joints adopted in machinery mechanisms. Both metric and screw threads produced by two methods (machining and rolling) are applied in filters. In both methods similar tools in form of screw-taps are used which are inserted into a previously prepared hole in the filter housing. Produced threads are practically identical and interchangeable concerning the dimensions. The only difference is that the machining screw-tap removes the surplus of the metal from the hole by cutting the particular thread rolls, whereas the rolling screw-tap (crusher) forms the rolls by plastic metal deformation and dislocation of plasticized layers.
Both for machined and rolled threads, the hole made in the cover prepared for the thread should have a certain diameter so that after threading a suitable contour height as well as the diameter of the crests of the contour (inner thread diameter) is obtained.
The hole for the thread should be prepared in such a way that obtaining the thread height contour of 60%-75% of theoretical size is provided.
Producing threads with the contour height above 75% causes excessive load of the screw-tap resulting from friction forces, which results from quicker wear of the tool and greater possibility of its destruction during work. Threads produced in this way are usually hard to screw in, and sometimes it is simply impossible due to the tolerances of the making of the screw.
Performed studies and tests concerning the thread joints endurance showed, that it is sufficient when achieving 60% of the thread contour height and does not demonstrate further significant growth while increasing the contour height which would justify the increase in production costs.
While many people presume that machined threads have sharp crests, in fact the crest of contour of a proper screw-cut thread has a flat surface. For cutting thread with sharp crests, having the theoretical thread contour height of 100%, a cutting crests "V" shaped screw-tap sharp enough to cut metal would be necessary. Manufacturing such a tool would be very hard and expensive.
The thread contour height depends on the diameter of the prepared hole. For threads cut by machining, the diameter is at the same time the diameter of the crests (inner thread diameter).
If the diameter of the prepared hole would be the same as the screw-tap inner diameter, we would get a 100% thread contour height. However it is neither necessary nor desired, as explained earlier. However the diameter of the hole has to be larger than the thread crests theoretical diameter and still small enough to obtain thread with the contour height of 60%-75% of the theoretical size.
Rolled threads have many advantages in comparison with machined threads. The main advantage is that during rolling the threads, which is the plastic metal deformation in the hole in the cover, there are no shavings and splinters. While machining the thread some shavings, which can damage the thread surface as well as the tool itself during work, appear. What is more, the shavings appearing in the process of machining, have to be removed carefully from the whole cover before mounting the filter. That's because they could get through to the engine with the oil flow and damage the surfaces of pans, cylinders or bearings.
Lack of shavings makes the threads machining process less sensitive to blockages and crackings of the screw-taps, which designed with no undercuts providing the working surface for the machining blade, making the crusher (name of screw-tap for rolling) a more resistant tool than machining screw-taps. What is more, the crushers, having no blade, cannot get blunt. For these reasons the life time of crushers is from 3 to 20 times longer and the process of rolling proceeds 1.5 to 2x faster than machining.
|Machining screw-tap||Rolling screw-tap|
Dissimilarity of thread rolling technologies entails that opposite to machined threads the hole prepared for thread rolling does not have the diameter equal to inner diameter of a ready made thread. The hole prepared for thread rolling must have a specific diameter for this technology which is set for each size of thread and desired height of the thread contour after rolling.
During rolling, the thread contour in the hole is obtained through deformation of the surface layer, which due to pressure caused by a tool, plasticizes and fills voids between coils of the crusher making at the same time the thread contour. The thread obtained using this technology is much more resistant and durable, moreover during rolling the probability of making the thread too loose, which happens during thread machining when it is broken during bearing slackness of the machine tool spindle is significantly reduced.
A characteristic feature of rolled threads is a concave layer of thread laps crests resulting from plastic deformation of metal layers in the hole. It happens that the said concave is interpreted as thread damage by third persons used to flat surface of crests obtained during thread machining.
|Outline of cut thread||Outline of rolled thread|
Prepared on the basis of Filter Manufacturers Council – Technical Service Bulletin 94-3R1