BACKGROUND OF THE INVENTION
[0001] The present invention relates to a method of treating the surface of a poppet valve.
[0002] As shown in Fig. 3, a poppet valve 3 made of heat resistant steel comprises a valve
head 1 and a valve stem 2, which is slidably engaged in a valve guide 4 which is a
press fit in a cylinder head(not shown). The valve 2 is driven by pressing its end
by a rocker arm 5 or a tappet(not shown).
[0003] The valve stem 2 always moves up and down at high speed along the valve guide 4 during
running of an internal combustion engine, and a lip seal 6 prevents excessive lubricating
oil from leakage onto the valve stem 1, so that the valve 3 is subject to severe frictional
condition.
[0004] Therefore, on the surface of the valve stem 2 in which desired surface roughness
is formed by grinding steps in the conventional valve 3, soft nitriding is applied
to form hardened layer, or Cr or Ni metal plating is applied, thereby increasing wear
resistance of the valve stem 2.
[0005] However, when the surface treatment, especially soft nitriding, is applied to the
valve stem 2 in the valve 3, the valve stem 2 has relative large amplitude surface
roughness as shown in Fig. 4, and there are rigid particles 7 such as coal and nitride
on the surface.
[0006] To overcome the disadvantages, it is suggested to grind a portion by a grinding wheel
in a grinder to decrease surface roughness, the portion slidably contacting the lip
seal 6 in the stem 2 to which surface treatment such as soft nitriding is applied.
But, the thickness of the hardened layer is too small to provide wear resistance and
durability, so that the thickness of the hardened layer must be previously formed
larger, taking thickness to be ground into consideration, so that surface treatment
takes a lot of time to lead lower productivity, or material for the treatment requires
increased cost, which is disadvantageous.
[0007] In view of the disadvantages, it is an object of the present invention to provide
a method of treating the surface of a poppet valve in which surface roughness of the
valve stem is easily decreased to keep wear of a lip seal as little as possible without
grinding a hardened layer of the surface.
[0008] To achieve the object, there is provided a method of treating the surface of a poppet
valve which comprises a valve head and a valve stem, the method comprising the steps
of:
forming a hardened layer on the surface of the valve stem;
pressing the surface of at least a portion of the hardened layer by pressure members
having flat pressing surfaces, the portion slidably contacting a lip seal.
[0009] The hardened layer may be prefarably formed by soft nitriding.
[0010] The hardened layer formed on the surface of the valve stem is pressed by the pressure
members, so that crests which have larger amplitudes on the surface of the hardened
layer are collapsed to become flat surfaces having smaller surface roughness. Thus,
the lip seal contacts the valve stem in area, so that area pressure of the contact
portion decreases.
[0011] Surface roughness of the hardened layer of the valve stem increases, so that wear
rate of the lip seal decrease and durability increases, thereby preventing oil from
dropping at the lip seal. Thus, oil standing and increase in consumption of lubricating
oil are avoided. Grinding of the hardened layer is not required, so that wear resistance
does not decrease. Surface roughness of the hardened layer decreases by relatively
simple means, thereby increasing productivity and decreasing production costs.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] The features and advantages of the present invention will become more apparent based
on the following description with respect to the accompanying drawings wherein:
Fig. 1 shows the steps of one embodiment of the present invention, (A) being a longitudinal
sectional view in which a hardened layer is formed on a poppet valve, (B) being a
longitudinal sectional view in which a portion which slidably contacts a lip seal
is pressed by a pair of pressure rollers;
Fig. 2 is an enlarged sectional view which shows surface roughness of the pressed
portion ;
Fig. 3 is a partially sectioned longitudinal front elevational view which illustrates
a valve operating mechanism to which the present invention is applied; and
Fig. 4 is an enlarged sectional view which shows surface roughness of a conventional
poppet valve.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENT
[0013] Fig. 1 (A) and (B) illustrate the steps of the present invention. A heat resistant
steel valve 10 was finished to desired size by a number of mechanical processing steps,
and a valve stem 10a, a valve head 10b and a cotter groove 10c of the valve 10 are
formerly ground except a valve face 10d by a grinding wheel of a grinder to have a
desired surface roughness such as 1.0 to 10.0µm as Rz. The pretreatment steps are
not shown. Then, as shown in Fig. 1(A), the surface except the valve face 10d is subject
to salt bath soft nitriding to form a hardened layer 11.
[0014] The hardened layer 11 is formed by heating a treatment bath which contains KCN, KOCN
and Na
4 [Fe(CN)
6] to about 600 C, and the formerly treated portions of the valve 10 is immersed in
the bath for a predetermined time. The thickness of the hardened layer 11 may be preferably
10 to 20µm.
[0015] Then, as shown in Fig. 1(B), a portion which slidably contact a lip seal (in Fig.
3) in the valve stem 10a having the hardened layer 11 is put between a pair of pressure
rollers 12 and 12 having flat pressing surfaces, and the surface of the hardened layer
11 is pressed with a fixed load such as 5kg/cm
2, while the rollers 12 rotate with the valve 3. The surface of the hardened layer
11 is changed to have flat surfaces having smaller surface roughness by collapsing
crests having larger amplitudes. The lip seal substantially contacts the valve stem
10a in area, so that pressure of the contact surface decreases, thereby decreasing
wear rate of the lip seal greatly to prevent oil from dropping at the lip seal. Roots
14 in the roughness of the surface act as oil standing, thereby increasing oil storage
of lubricating oil.
[0016] In the foregoing embodiment, only the portion which slidably contacts the lip seal
is pressed by the pressure rollers 12 , but the pressure rollers may press all the
surface of the valve stem which slidably contacts a valve guide (as shown in Fig.
3), or the surface of the hardened layer 11 which slidably contacts the upper and
lower ends of the valve guide, thereby decreasing wear rate of the valve guide as
well as the above.
[0017] In order to form the hardened layer 11 in the step of Fig. 1(A), instead of the salt
bath soft nitriding, gas nitriding, ionized nitriding, gas cementation nitriding,
metal plating, coating and spraying may be used. Means for forming the hardened layer
11 in the present invention are not limited to soft nitriding so far as surface roughness
becomes relatively small after formation of the hardened layer 11. The present invention
may also apply to Ti or Ti alloy valve as well as the heat resistant steel one.
[0018] The foregoings merely relate to embodiments of the present invention. Various changes
and modifications may be made by person skilled in the art without departing from
the scope of claims wherein:
1. A method of treating a surface of a poppet valve which comprises a valve head and
a valve stem, the method comprising the steps of:
forming a hardened layer on the surface of the valve stem;
pressing the surface of at least a portion of the hardened layer that is intended,
in use, to make sliding contact with a lip seal, by pressure members having flat pressing
surfaces.
2. A method as defined in Claim l wherein the hardened layer is formed by soft nitriding.
3. A method as defined in Claim l or 2 wherein the pressure members comprises a pair
of pressure rollers by which crests of a roughened surface of the said portion of
the valve stem are collapsed to decrease surface roughness and thereby to prevent
oil standing.