Valve seat crack detection
Powder metal can be cause of valve seat crack
One of the most defects that appear during the engine assembly are the cracks on the valve seat inserts or on the head engine closely the spark plug holes. Both defects can be procured by different causes as: wrong material density, wrong insertion procedures, misalignment of the assembled parts, wrong dimensional or geometrical tolerances and more. Cracks during the assembly appear not for stress fatigue but for frail materials. Cast engine head and some kind of valve seat are made in materials that can include cavities generated during the manufacturing process, these cavities aren't visible at first glance through a visual inspection but need expensive x ray controls to be detected. Nowadays the new casting process can reduce or eliminate at the 99% the possibility of cavities, also the powder metal seats are very good, the homogeneous consistency of a powder metal provide an extend engine durability, but sometime the powder metal materials used to manufacturing the new generation of valve seat can hidden problems of cavities or not homogeneous compound that became a source of crack as soon as the compressive force is actuated for insertion. Cavities aren't the lonely causes for cracks in valve seat insert, the powder metal composite used to manufacturing the valve seat hasn't a good elasticity and pass from elastic behaviour to the rupture as soon as overcame the strength limit. This means that also high quality powder metal valve seat can be cracked if won't be properly inserted. The use of the correct tools and correct procedure in valve seat insertion is the right way to reduce the scraps, in fact the use only of x ray to inspects the valve cracks could be useful only to seek them, but not to improve the insertion process and reduce the scraps costs.
Criticism generated by narrow tolerances in valve seat insertion
The dimensional interference between the machined bore to allocate the valve seat and the external diameter of the valve seat it is very narrow, this to avoid any lackage for thermal dilatation during engine combustion. The seats in some OEM heads may have as little as 0.05 mm of interference fit, but the new trend tend to use more interference fit to compensate for any misalignment in the seat valve locations, in this case the interference can be increased until of 0.1mm in cast iron heads, and up to 0.17mm of interference fit in aluminum heads.The extreme narrow interference goes to create a strong pressure against the external ring of the valve seat during the insertion. This compressive stress could be causes of crack during the insertion in case of bad compounded material. Cooling the seat valve by liquid nitrogen can be a good solution to reduce the interference during the insertion but it is also a way to increase the frail of the valve seat insert. Often is not possible avoid the use of nitrogen to reduce the interference, an aluminium head engine for example own a good elastic behaviour and at first glance this could be useful to expand the valve seat location during the insertion, but the aluminium thermal expansion need also of more interference to avoid valve lackage when the engine run. More interference means more strength during the valve seat insertion and the possibility that the machined bores in the engine head results extruded by the valve seat, sometime your valve seat insertions wise well blocked into the head engine, but in reality is only gripped into an extruded hole. A good practices during the valve seat insertion is the lubrication of the valve seat by appropriate grease developed for this scope to avoid harmful smokes and carbon particles in the engine head at the first combustion. How you can see, valve seat insertion need of a lot of care during the press fit process and all solutions applied to improve this process could hidden some critical factors.
Criticism of the press fit process in seat valve insertion
If we going to examine the valve seat insertion, the main critical step comes when the valve seat is full compressed radially and approach the mechanical stop machined at the end of the bore in the engine head (see the picture below).
In this case the integrity of your valve seat is still right, but as soon as touch the mechanical stop an axial compressive force will be added to the radial compression and this combined stress is one on the main cause for valve seat crack. The force generated when a part is compressed against a mechanical stop increase with an exponential behaviour, in process like valve seat insertion manage this force became very important to avoid cracks. The use of hydraulic or pneumo-hydraulic actuators can't provide a precise control of the force generated by a part against a mechanical stop, this because sometime aren't controlled by a force monitoring or in case this kind of devices are controlled by a pressing monitoring, this results not enough faster to cut off the power as soon as the force increase exponentially.
How Auloma's servo press solve the problems of valve seat crack during the insertion
The use of a properly servo press solve this problem at the root. In the market exist a lot of servo presses but few are capable of seek the valve seat crack. Auloma's servo presses mount very sensitive sensors and through its advanced software's algorithms are capable to analyse the force variations caused by the crack. Further this capability, the use of a servo press enhance the valve seat insertion process. The servo motor used to control the servo press provide a fine control of the press ram optimized to insert the valve seat insert with the best parameters of speed, acceleration and dwell time. A properly drive profile to control the press ram movement during the insertion, give at the valve seat insert the time to out stretching the material structure and avoid cracks procured by excessive Joule in the press fit job. Auloma's servo presses have installed in the servo drive the software for press control and process monitoring and analyses directly the signals provided by the sensors installed in the servo press. The direct acquisition of the servo press signals provide a real time process analysis and improve the valve seat insertion process. The faster control in few milliseconds bear the immediate stop of the servo press ram as soon as the valve seat insert touch the mechanical stop. Press ram blocked faster as soon as the contact force start to increase exponentially, means avoid valve seat crack for excessive compressive force, but with the confidence that your valve seat insert is really against the mechanical stop.
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Auloma servo press benefints in valve seat insertion and crack detection
Have a servo press with powerful process monitoring embedded in the servo drive mean known in each step of the valve seat insertion if appear force variations that means the presence of crack and thanks to its capabilities to manage the press ram during the insertion it will be possible prevent the cracks for not properly insertion.
- Monitoring force insertion and seek variations that means presence of cracks
- Control of the compressive force when the valve seat touch the mechanical stop in its location to avoid crack for excessive stress
- Insertion on the valve seat insert with properly press ram speed to avoid excessive stress during ty the job
- Scrap reduction
- Detection of all cracks procured during the insertion