WO2017007397A1

WO2017007397A1 - A rocker arm and a rocker arm assembly

Info

Publication number: WO2017007397A1
Application number: PCT/SE2016/050601
Authority: WO
Inventors: Linnéa WESSMAN; Johan FAHLKRANS; Thorbjörn Hansen
Original assignee: Scania Cv Ab
Priority date: 2015-07-03
Filing date: 2016-06-22
Publication date: 2017-01-12
Also published as: SE1550958A1

Abstract

A rocker arm (1) and a rocker arm assembly, wherein the rocker arm has a through-hole for rotatably receiving a support shaft (2). The rocker arm is manufactured from steel and the through- hole is delimited by an outer surface (3) of a bearing surface zone comprising an elevated amount of nitrogen and having an increased hardness with respect to a steel matrix below the bearing surface zone. The support shaft is preferably manufactured from ductile cast iron.

Description

A rocker arm and a rocker arm assembly

TECH N ICAL FI ELD OF TH E I NVENTION

The present invention relates to a rocker arm according to the preamble of claim 1 and a rocker arm assembly according to the preamble of claim 8. Such a rocker arm and rocker arm assembly are used in motor vehicles, in particular in vehicles comprising an internal combustion engine.

BACKG ROU N D AN D PRIOR ART

A conventional i nternal combustion engine of piston type, a so- called piston engine, is provided with one or more cylinders. I n each cylinder a piston is configured to reciprocate under the action of the pressu re which occurs upon ignition of a fuel-air mixtu re introduced into the cyli nder. I n a piston engine of fou r- stroke type, each cylinder is provided with at least one inlet valve via which the fuel-air mixture is drawn into the cylinder, and at least one exhaust valve via which combustion gases are removed from the cylinder. For this purpose, the inlet valve and the outlet valve are movable between a closed an open position . The valves may also be actuated for other reasons than for the inlet and outlet related to combustion. An example of this fu nction is decompression braking, in which the valves are engaged without combustion to obtain a retardi ng effect of the crankshaft. The inlet valve and the exhaust valve are each spri ng-loaded to a closed position and are each movable from closed to open position by the action of a control cam on a rotati ng camshaft. The control cam is normally configu red to act upon the valve via a so-called rocker arm, which is provided with a th roug h-hole for rotatably receiving a support shaft or a bearing bracket. The rocker arm is com monly manufactured from steel or cast i ron and the support shaft is com monly manufactured from steel. However, if the valves are to be used also for decompression braking, the demands on the mechanical strength of the rocker arm increases and it is usually not possible to use a cast iron rocker arm .

A plain bearing is usually provided between the support shaft and the th roug h-hole to mini mise the risk of seizure and wear du ring relative motion of the parts arising during relative rotational motion. Such plain bearings used between the rocker arm and the support shaft include sleeve bearings typically comprising leaded bronze, providing a lubricati ng film of lead at the bearing interface. However, for environ mental reasons and i n order to meet increasingly strict legal requirements regardi ng leaded products, it is desirable to develop an alternative solution to the use of such bearings. Fu rthermore, bronze sleeve beari ngs, including lead-free such bearings, exhibit an impai red performance in acid environments. Modern engine oils have less ability to buffer acids, resulting from oxidation of the engine oil and contamination via condensation and combustion by-products. Thus, the performance of the bearings is negatively affected , especially for lead-containing bearings and bearings based on bronze or other yellow metals. A fu rther problem with bronze sleeve bearings, in particular for heavy motor vehicles such as trucks and buses, is that hig h wear arises when the vehicle is equipped with a so-called stop-start system. Such a system is increasingly common and automatically shuts down the combustion engine when the vehicle is stopped in order to save fuel and to reduce emissions.

SUMMARY OF TH E I NVENTION

It is a primary objective of the present invention to provide a rocker arm and a rocker arm assembly which are in at least some aspect improved with respect to known such components. I n particular, it is an objective to provide a rocker arm and a rocker arm assembly by means of which the above mentioned problems are overcome. Another objective is to provide a rocker arm and a rocker arm assembly with a mechanical strength such that the rocker arm and the rocker arm assembly are suitable for use in con nection with decompression brakes.

At least the primary objective is, according to a first aspect of the present invention, achieved by a rocker arm having a th rough- hole for rotatably receiving a support shaft, wherein the rocker arm is manufactured from steel and wherein the th rough-hole is delimited by an outer su rface of a bearing surface zone, which is characterised in that the bearing surface zone comprises an elevated amou nt of nitrogen and has an increased hardness with respect to a steel matrix below the bearing su rface zone. The hardened surface zone at least arou nd the th rough-hole together with the steel core of the rocker arm provides a combination of core tough ness and su rface hardness. The rocker arm has a high mechanical strength and is thereby suitable for bei ng used in connection with compression braking. The hardened surface zone provides increased wear resistance and resistance to seizu re in the bearing interface between the rocker arm bearing su rface zone and a contact su rface of a support shaft mou nted in the through-hole, and it also reduces the friction arising upon relative movement of the parts. Such a surface zone with increased hardness can be in the form of a conversion coating formed on the steel matrix of the rocker arm. The hardened surface zone may, apart from nitrogen , also comprise an elevated amount of carbon .

The steel should be a steel suitable for being subjected to a nitriding or a nitrocarbu rising process, such as a steel for quenching and tempering with a typical hard ness i n the range between 1 50 to 350 HV. The steel is preferably such that a compound layer is provided when the steel is subjected to a nitriding process. The hardness of the bearing surface zone should preferably be at least 500 HV, and can be significantly hig her, such as 1 000 HV and above. According to one embodiment of the invention, the bearing surface zone has been formed in a nitriding process or in a nitrocarbu rising process. There are a plurality of different suitable nitridi ng and nitrocarburising processes which result in an increased hardness of the surface zone. Nitriding and nitrocarbu rising have the advantage over other surface hardening methods that the required process temperature is low, typically 400-650 °C, of which nitriding temperatu res are typically 480- 550 °C, and nitrocarbu rising temperatu res are typically 550- 650 °C. The rocker arm can thereby be nitrided in a fully hardened and tempered condition without the core properties of the steel being adversely affected. Nitriding generally diffuses nitrogen into the su rface zone, thus increasing the concentration of nitrogen , while nitrocarburising also diffuses carbon into the surface zone. It is possible to mask part of the rocker arm during the nitriding or nitrocarbu rising process if it is desired that only the surface zone delimiti ng the through-hole should be hardened. However, it is also possible to leave the rocker arm un masked and obtain a su rface layer of increased hardness on the entire rocker arm .

According to one embodiment of the invention, the bearing surface zone is a nitrided surface zone. Such a nitrided su rface zone can be achieved by e.g. gas nitriding and exhibits the desired properties regarding resistance to seizure, hard ness and wear resistance.

According to one embodiment of the invention, the bearing surface zone comprises an outer compou nd zone having a thickness within a range of 5-50 Mm. Such a compou nd zone, sometimes referred to as a "white layer", is a phase transformed layer achieved during a nitriding or a nitrocarbu rising process and provides excellent resistance to seizu re, wear resistance and corrosion resistance to the bearing surface zone. With a thickness of at least 5 M m, sufficient wear resistance and durability is achieved . Making the compou nd zone too thick requires very long processing times, and fu rthermore reduces the quality of the compou nd layer. Preferably, the outer compou nd zone has a thickness within a range of 1 0-20 m. According to one embodiment of the invention, a diffusion zone with an inward g radually decreasi ng hardness is formed between the outer compound zone and the steel matrix, said diffusion zone having a thickness within a range of 50-800 M m, preferably 1 00-400 Mm. Such a diffusion layer provides a g radual transition from the hig h hardness of the surface to the relatively low hardness of the steel matrix of the core and thereby increases the structu ral strength of the rocker arm. Preferably, the hardness of the diffusion zone at an interface toward the outer compound zone is at least 450 HV.

According to another aspect of the present i nvention, at least the above mentioned pri mary objective is achieved by means of a rocker arm assembly comprising a support shaft and at least one rocker arm as proposed, wherein the rocker arm is configured to be rotatably mounted around the support shaft. Advantages and advantageous embodi ments of such an assembly appear from the above description of the proposed rocker arm. The support shaft can be in the form of a long support shaft supporting several rocker arms, or in the form of one or several bearing brackets having an integrated shaft member, dependi ng on the design of the engine in which the rocker arm assembly is operating . Normally, the support shaft is fixed in the rocker arm assembly, and the rocker arm rotationally pivots arou nd the support shaft. According to one embodiment of this aspect of the i nvention , the outer su rface of the bearing su rface zone of the at least one rocker arm is in rotational contact with a peripheral surface of the support shaft.

According to one embodiment of this aspect of the i nvention , the support shaft is manufactured from cast iron . Preferably, the cast iron is in the form of ductile cast iron. Ductile cast iron, also referred to as e.g . spherulitic g raphite cast iron and nodular graphite cast iron, contains graphite which provides to some deg ree a lubricati ng film at the bearing interface between the rocker arm and the support shaft. The steel with the bearing surface zone provides resistance to seizu re in the bearing interface. The combination of the two materials provides a bearing interface without having to use an intermediate plain bearing component, such as a sleeve. This combi nation of materials thus provides excellent strength and hardness of the rocker arm i n combination with the lubricating properties of the support shaft. The ductile cast i ron preferably has a minimu m tensile strength of 350 M Pa and an elongation A5 of minimu m 3%. The elongation A5 is defined as the elongation up to the poi nt of fracture of a bar, measured over an initial gauge length of 5 times the bar diameter. Fu rther advantages and advantageous features of the invention will appear from the following detailed description .

BRI E F D ESC RI PTION OF TH E D RAW INGS The invention will now be described by way of example with reference to the appended drawings, in which :

Fig . 1 shows an exploded view of a rocker arm assembly accordi ng to an embodiment of the invention ,

Fig . 2 shows a perspective view of the rocker arm assembly in fig . 1 , and

Fig . 3 schematically shows a sectional view of a su rface zone of a rocker arm according to an embodiment of the invention.

D ETAILE D D ESC RI PTION OF EM BOD I M ENTS OF TH E INVENTION An exploded view of a rocker arm assembly according to an embodiment of the invention is schematically shown in fig . 1 . The rocker arm assembly comprises two rocker arms 1 and a support shaft 2, which extends along a central axis. Each of the rocker arms 1 comprises a th rough-hole for rotatably receiving the support shaft 2, such as shown in fig . 2. The rocker arms 1 can thereby rotate arou nd the support shaft 2. Fastening members 5 for fastening the support shaft 2 are provided outside of the rocker arms 1 .

The through-hole of each rocker arm is delimited by an outer surface 3 of a bearing su rface zone 4, schematically shown in fig . 3. The outer surface 3 is in direct contact with a peripheral surface 6 of the support shaft 2, so that a rotational plain bearing is formed by the outer surface 3 of the bearing su rface zone 4 of the rocker arm 1 and the peripheral surface 6 of the support shaft 2.

The rocker arms 1 are manufactu red from steel and are, after hardening and tempering, preferably su rface hardened by a nitriding or nitrocarbu rising process. The steel grade should therefore be suitable for bei ng subjected to such a process, such as a steel for quenching and tempering with a typical hard ness in the range between 1 50 to 350 HV. Suitable steel grades for this purpose fulfill the requirements listed in Swedish standard SS- EN 1 0083. The support shaft 2 is preferably manufactu red from ductile cast iron, i.e. cast iron containing graphite which can act as a lubricant between the peripheral su rface 6 and the outer surface 3 of the bearing su rface zone 4.

The beari ng surface zone 4 of the rocker arm 1 , in comparison with a steel matrix 9, comprises an elevated amount of nitrogen, and in some cases also of carbon , as a result of the nitriding or nitrocarbu rising process. The bearing surface zone 4, as shown in fig . 3, comprises an outer compound zone 7 having a hardness of at least 500 HV, as compared to the core hardness of the steel matrix 9 of the rocker arm 1 of between 1 50 and 350 HV. The hardness of the compound zone 7 depends on the steel grade used, wherein high-alloy steel g rades generally give a compound zone of higher hardness, such as 1 000 HV or above. The compound zone typically consists of non-metallic phases formed by iron and nitrogen, such as γ-phase iron nitride (Fe₄N), containing up to 6 wt. % N , and ε-phase (Fe₂-3 N ) containi ng up to 1 1 wt. % N . I n the case when the bearing surface zone 4 has been formed in a nitrocarburising process, the compou nd zone 7 also comprises iron-carbonitrides. Between the steel matrix 9 and the compound zone 7, a diffusion zone 8 is formed . The diffusion zone 8 has a hard ness which g radually decreases from the compou nd zone 7 and toward the steel matrix 9. The hardness of the diffusion zone 8 at the interface toward the outer compound zone 7 should at least be 450 HV. The diffusion zone contains nitrogen in solid solution as well as stable metal nitrides formed by alloying elements of the steel. The thickness of the compound zone 7 is typically i n the range 5-50 m, preferably 1 0-20 m. The thickness of the compound zone 7 increases with processing time and the ideal thickness is therefore dependent on desired lead times in the production as well as on desired material properties of the rocker arm 1 . The diffusion zone 8 typically has a thickness withi n a range of 50- 800 M m, preferably 1 00-400 Mm.

The nitridi ng or nitrocarburising process used to form the bearing surface zone 4 can be any suitable process used in the field, such as salt bath nitrocarbu rising, gas nitriding, gas nitrocarbu rising , low pressure nitriding, low pressure nitrocarbu rising , plasma nitriding, or plasma nitrocarbu rising .

The rocker arm assembly may of cou rse comprise a different number of rocker arms than described above, and the support shaft may be accordingly configured. The support shaft can e.g. be in the form of one or more bearing brackets with an integrated shaft member, or a long shaft configured to hold several rocker arms depending on the design of the engi ne in which the rocker arm assembly is mou nted . The invention is of cou rse not in any way restricted to the embodiments described above. On the contrary, many possibilities to modifications thereof will be apparent to a person with ordinary skill i n the art without departing from the basic idea of the invention such as defined in the appended claims.

Claims

1. A rocker arm (1) having a through-hole for rotatably receiving a support shaft (2), wherein the rocker arm (1) is manufactured from steel and wherein the through-hole is delimited by an outer surface (3) of a bearing surface zone (4), wherein the bearing surface zone (4) comprises an elevated amount of nitrogen and has an increased hardness with respect to a steel matrix (9) below the bearing surface zone (4),

characterised in

that the bearing surface zone (4) has been formed in a nitrocarburising process, and in that the bearing surface zone (4) comprises an outer compound zone (7) comprising iron nitrides and iron carbonitrides and having a thickness within a range of 5-50 Mm.

2. The rocker arm according to claim 1, wherein the outer compound zone (7) has a thickness within a range of 10-20 m.

3. The rocker arm according to claim 1 or 2, wherein a diffusion zone (8) with an inward gradually decreasing hardness is formed between the outer compound zone (7) and the steel matrix (9), said diffusion zone (8) having a thickness within a range of 50-800 Mm.

4. The rocker arm according to claim 3, wherein the diffusion zone (8) has a thickness within a range of 100-400 Mm.

5. A rocker arm assembly comprising a support shaft (2) and at least one rocker arm (1) according to any one of the preceding claims, wherein the rocker arm (1 ) is configu red to be rotatably mou nted around the support shaft (2) .

6. The rocker arm assembly accordi ng to claim 5, wherein the outer surface (3) of the bearing su rface zone (4) of the at least one rocker arm (1 ) is in rotational contact with a peripheral surface (6) of the support shaft (2) .

7. The rocker arm assembly according to claim 5 or 6, wherein the support shaft (2) is manufactured from cast iron.

8. The rocker arm assembly accordi ng to claim 7, wherein the cast i ron is in the form of ductile cast iron.