US20200190616A1

US20200190616A1 - Sliding camshaft and method of manufacturing

Info

Publication number: US20200190616A1
Application number: US16/222,079
Authority: US
Inventors: Richard L. Metcalf
Original assignee: GM Global Technology Operations LLC
Current assignee: GM Global Technology Operations LLC
Priority date: 2018-12-17
Filing date: 2018-12-17
Publication date: 2020-06-18

Abstract

A sliding camshaft and a method of manufacturing a lobe pack for a sliding camshaft are provided. The lobe pack is formed from a steel alloy having a carbon content in the range of 0.48-0.53 wt %. The lobe pack is heated to a predetermined austenitizing temperature that is below a carburizing temperature of the steel alloy for a time period at least long enough to heat the entire lobe pack to the predetermined austenitizing temperature. The lobe pack is quenched to a martempering temperature that is greater than a martensite start temperature and within 50 degrees Celsius of the martensite start temperature. The lobe pack is held at the martempering temperature for a martempering time period that is long enough to cool the entire lobe pack to the martempering temperature. The lobe pack may have a hardness in the range of 56 to 60 HRC throughout the lobe pack.

Description

TECHNICAL FIELD

The disclosure generally relates to a sliding camshaft assembly and a method of manufacturing a lobe pack of a sliding camshaft for an internal combustion engine.

INTRODUCTION

Some internal combustion engines include an adjustable or slideable camshaft assembly. The sliding camshaft assembly includes an inner rod or shaft that is rotatable about a cam central axis, and a sliding camshaft (including one or more lobe packs) that is slidably attached to the inner rod for axial movement along the cam central axis relative to the inner rod. The sliding camshaft is rotatable with the inner rod about the cam central axis. The sliding camshaft is moveable between at least two different axial positions along the cam central axis, relative to the inner rod. Each different position of the sliding camshaft presents a different cam lobe having a different lobe profile for engaging a respective valve stem of the engine. Accordingly, by adjusting the position of the sliding camshaft along the cam axis relative to the base camshaft, the cam profile that each valve stem of the engine follows may be changed. It is known to form lobe packs of the sliding camshaft from a high carbon steel, such as AISI 52100 alloy steel. However, when these lobe packs are vacuum hardened to bring them to a hardness required for internal combustion engine applications, the vacuum hardening process causes distortion in the shape of the camshaft, which often requires additional material removal by hard broaching.

SUMMARY

The present disclosure provides a sliding camshaft and a method for forming a lobe pack of a sliding camshaft that includes martempering a lobe pack formed of 5150 steel to harden the entire lobe pack, which minimizes distortion, lowers residual stresses, and improves machinability, while maintaining acceptable Hertzian contact load capability.
In one form, which may be combined with or separate from other forms provided herein, a method of manufacturing a lobe pack for a sliding camshaft of an internal combustion engine is provided. The method includes providing the lobe pack formed from a steel alloy having a carbon content in the range of 0.48 to 0.53 weight percent. The method also includes heating the lobe pack to a predetermined austenitizing temperature that is below a carburizing temperature of the material for a heating time period. The heating time period is at least long enough to heat the entire lobe pack to the predetermined austenitizing temperature. The method further includes quenching the lobe pack to a martempering temperature. The martempering temperature is greater than a martensite start temperature and within 50 degrees Celsius of the martensite start temperature. The method includes holding the lobe pack at the martempering temperature for a martempering time period. The martempering time period is at least long enough to cool the entire lobe pack to the martempering temperature.
In another form, which may be combined with or separate from other forms disclosed herein, a sliding camshaft for an internal combustion engine is provided. The sliding camshaft includes an inner rod extending along a cam axis and a lobe pack circumferentially disposed about the inner rod. The lobe pack has an inner side disposed in sliding contact with the inner rod and an outer surface bearing a number of cam lobes. The lobe pack is formed of a steel alloy having a carbon content in the range of 0.48 to 0.53 weight percent. A majority of the microstructure of the lobe pack is martensite. The lobe pack has a hardness of at least 56 HRC on the Rockwell hardness scale throughout the lobe pack.
Additional features may be provided, including but not limited to the following: wherein quenching to the martempering temperature is performed directly after the step of heating, without heating the lobe pack above the carburizing temperature of the lobe pack material; the steel alloy having a manganese content in the range of 0.70 to 0.90 weight percent; the steel alloy having a silicon content in the range of 0.15 to 0.35 weight percent; the steel alloy having a chromium content in the range of 0.70 to 0.90 weight percent; the steel alloy including phosphorus in an amount not exceeding 0.030 weight percent; the steel alloy including sulfur in an amount not exceeding 0.040 weight percent; the steel alloy being a 5150 steel alloy; the martempering temperature being in the range of about 285 to about 330 degrees Celsius; the predetermined austenitizing temperature being at least 800 degrees Celsius; the carburizing temperature being at least 900 degrees Celsius; the martempering time period being at least 25 minutes; the heating time period being at least 45 minutes; wherein the steps of quenching the lobe pack to the martempering temperature and holding the lobe pack at the martempering temperature include submersing the lobe pack within a salt bath; the method further including cooling the lobe pack to an ambient temperature after holding the lobe pack at the martempering temperature for the martempering time period; and wherein the lobe pack has a hardness of no greater than 60 HRC on the Rockwell hardness scale throughout the lobe pack.
Further aspects, advantages and areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples and drawings are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a cam system having a sliding camshaft including a plurality of lobe packs for an engine of a motor vehicle, according to the principles of the present disclosure;

FIG. 2 is a cross-sectional view of a portion of a lobe pack of the cam system of FIG. 1, in accordance with the principles of the present disclosure;

FIG. 3 is a block diagram illustrating a method for manufacturing a lobe pack of a sliding camshaft, according the principles of the present disclosure; and

FIG. 4 is a graph illustrating a heat-treating profile for a lobe pack of a sliding camshaft, in accordance with the principles of the present disclosure.

DETAILED DESCRIPTION

Those having ordinary skill in the art will recognize that terms such as “above,” “below,” “upward,” “downward,” “top,” “bottom,” etc., are used descriptively for the figures, and do not represent limitations on the scope of the disclosure, as defined by the appended claims. Furthermore, the teachings may be described herein in terms of functional and/or logical block components and/or various processing steps. It should be realized that such block components may be comprised of any number of hardware, software, and/or firmware components configured to perform the specified functions.
Features shown in one figure may be combined with, substituted for, or modified by, features shown in any of the figures. Unless stated otherwise, no features, elements, or limitations are mutually exclusive of any other features, elements, or limitations. Furthermore, no features, elements, or limitations are absolutely required for operation. Any specific configurations shown in the figures are illustrative only and those shown are not limiting of the claims or the description.
As used herein, the term “hardness” is defined as a measure of how resistant a workpiece is to various kinds of permanent shape change when a compressive force is applied.
In the drawings, like reference numbers correspond to like or similar components whenever possible throughout the several figures. Referring now to FIG. 1, a cam system is schematically illustrated and generally designated at 10. The cam system is configured to operate at least one first engine valve 12 and at least one second engine valve 14 of an internal combustion engine (not shown) of a motor vehicle. The first engine valves 12 and the second engine valves 14 are located within respective first and second cylinders (not shown) or cylinder banks of the internal combustion engine. The cam system 10 shown may also be used on a single cylinder.
A sliding camshaft 16 is supported by a plurality of bearings 18, such that the sliding camshaft 16 is rotatable relative to the cylinder banks. As described herein, rotation of the sliding camshaft 16 variably actuates the first engine valves 12 and the second engine valves 14 to facilitate combustion within the cylinder banks and production of mechanical energy by the engine. Additional bearings 18 may be incorporated into the sliding camshaft 16.
The first engine valves 12 and the second engine valves 14 are poppet valves used to control the timing and quantity of fuel and air flow into the engine. The structures of the cylinders, such as valve seats, cylinder walls, etc., are omitted from the figures.
Although the cam system 10 is described and illustrated with reference to the technology of internal combustion engines, which includes spark-ignited and compression ignited engines, the structures and functions of the cam system 10 are usable in other technological areas. The cam system 10 is applicable to any cam-driven valve technology, including valves used to control flow of other fluids or solids. For example, plastic molding equipment may utilize iterative injection of solid or liquid plastics into molds. Additionally, the cam system 10 may be used to control other iterative structures, technologies, or assemblies, such as actuating manufacturing devices. In general, cams provide iterative and controllable physical actuation from rotating movement, and the cam system 10 may be utilized with any such system.
As shown in FIG. 1, the cam system 10 includes a shaft or inner rod 20, which is rotatable relative to the first engine valves 12 and the second engine valves 14, but which does not otherwise translate horizontally or move vertically (as viewed in the figures). A first sliding lobe pack 22 is translatable relative to the first engine valves 12 and the inner rod 20, and a second sliding lobe pack 24 is translatable relative to the second engine valves 14 and the inner rod 20. The first sliding lobe pack 22 and the second sliding lobe pack 24 may be splined, or otherwise keyed, for common rotation with the inner rod 20. Each of the lobe packs 22 is circumferentially disposed about the inner rod 20, with an inner side 29 of each lobe pack 22, 24 in sliding contact with the inner rod 20. The outer side 31 of each lobe pack 22, 24 bears the cam lobes 26, 27, 28.
The first sliding lobe pack 22 is configured to operate the first engine valves 12 with one of a high-lift lobe 26, a low-lift lobe 27, and a zero-lift lobe 28. The second sliding lobe pack 24 is configured to operate the second engine valves 14 with one of a high lift lobe 26 or either of two low-lift lobes 27, which impart substantially identical displacement, and may be combined into a single low-lift lobe 27 in some configurations. Any of the individual lobes described herein and shown in the figures may be referred to numerically, as first, second, third, or the like. Although the lobe packs 22, 24 are shown having three different lift heights, in the alternative, the lobe packs 22, 24 could provide only two lift heights, or four or more lift heights, without falling beyond the spirit and scope of the present disclosure.
As described herein, translation of the first sliding lobe pack 22 and the second sliding lobe pack 24 selectively aligns the high-lift lobes 26, the low-lift lobes 27, and the zero-lift lobes 28 with the first engine valves 12 and the second engine valves 14. Alignment of the specific lobes 26, 27, 28 selectively varies the displacement of the first engine valves 12 and the second engine valves 14.
The lobes 26, 27, 28 are illustrated only schematically in the figures, such that the lobes 26, 27, 28 shown represent only relative maximum displacement of the first engine valves 12 and the second engine valves 14. Therefore, the high-lift lobes 26 impart greater motion to the first engine valves 12 and the second engine valves 14 than the low-lift lobes 27. The zero-lift lobes 28 impart substantially no lift to the first engine valves 12, such that the first engine valves 12 may be selectively deactivated.
The first sliding lobe pack 22 and the second sliding lobe pack 24 may be separately translatable relative to the first engine valves 12 and the second engine valves 14. However, in the configuration shown, the first sliding lobe pack 22 and the second sliding lobe pack 24 are connected for common translation by a shift barrel 30 that is attached to the first sliding lobe pack 22 and the second sliding lobe pack 24. The shift barrel 30 has a first groove 32 and a second groove 34, which join into a common groove 36.
Based on alignment of the high-lift lobes 26, the low-lift lobes 27, and the zero-lift lobes 28 relative to the first engine valves 12 and the second engine valves 14, the cam system 10 operates at a plurality of variable cam stages or states, including: a high-lift state, a low-lift state, and a cylinder deactivation or active fuel management state. Each of the operating states varies the amount of air and fuel entering the first cylinder bank and the second cylinder bank, which varies the operation of the engine.
A shift actuator 40 is fixedly disposed adjacent the shift barrel 30. The shift actuator 40 has a first pin 42, a second pin 44, and a third pin 46. The shift actuator 40 selectively deploys, fires, or actuates the first pin 42, the second pin 44, and the third pin 46, which may then engage one of the first groove 32 and the second groove 34 of the shift barrel 30. The first groove 32 and the second groove 34 are configured to act in opposing directions. Therefore, firing one of the pins into the first groove 32 translates both the first sliding lobe pack 22 and the second sliding lobe pack 24 in a first direction (leftward, as illustrated in FIG. 1); and firing one of the pins into the second groove 34 translates both the first sliding lobe pack 22 and the second sliding lobe pack 24 in the opposite direction (rightward, as illustrated in the figures). In the configuration shown, the shift actuator 40 is a single actuator for all of the first engine valves 12 and the second engine valves 14, as opposed to having one actuator related to the first cylinder bank and another related to the second cylinder bank. It should be understood, however, that any other configuration for the actuator 44 could alternative be used.
A control system or controller 50 is in communication with the shift actuator 40. The controller 50 is configured to instruct or cause the shift actuator 40 to actuate one of the first pin 42, the second pin 44, and the third pin 46. The controller 50 may also monitor the current state of the cam system 10, and may be involved with determining which of the states is currently preferred for operation of the vehicle.
In order to provide superior wear resistance and durability, each lobe 22, 24 of the sliding camshaft 16 is formed to have a hardness equal to or greater than HRC 56 on the Rockwell hardness scale throughout the depth of each lobe pack 22, 24, from the outer surfaces 31 to the inner sides 29, and even at the thickest wall locations 33. In some examples, the hardness of each lobe pack 22, 24, throughout their entire depths, is in the range of 56-60 HRC on the Rockwell hardness scale. In other cases, the HRC may exceed 60 HRC. Additionally, the process described herein produces very little or no distortion in the geometric shape of the lobe packs 22, 24 during the manufacture and heat treatment thereof.
Each lobe pack 22, 24 is formed of a steel alloy having a carbon content in the range of 0.48 to 0.53 weight percent. The steel alloy may further include a manganese content in the range of 0.70 to 0.90 weight percent, a silicon content in the range of 0.15 to 0.35 weight percent, a chromium content in the range of 0.70 to 0.90 weight percent, a phosphorus content up to 0.030 weight percent, and a sulfur content up to 0.030 weight percent. In one example, each lobe pack 22, 24 is formed of 5150 steel.
Referring now to FIG. 3, a method of forming a lobe pack 22, 24 of the sliding camshaft 10 is schematically illustrated and generally designated at 100. The method 100 includes a step 102 of providing the lobe pack 22, 24 from a steel alloy having a carbon content in the range of 0.48 to 0.53 weight percent. As explained in the preceding paragraph, the steel alloy may also be provided with contents of manganese, silicon, chromium, phosphorus, and/or chromium, and in a preferred example, the steel alloy may be provided as a 5150 steel alloy. Providing the lobe pack 22, 24 may include forming each lobe pack, 22, 24 to define a desired shape using standard forming and/or milling processes, such as forging, milling, grinding, stamping, drilling, machining, etc., or other shaping and/or polishing techniques known for forming steel parts.
Once each lobe pack 22, 24 has been formed and/or shaped to define a desired shape, and each lobe pack is heat treated with a martempering heat-treatment process. Thus, the method 100 includes a step 104 of heating the lobe pack 22, 24 to a predetermined or selected austenitizing temperature that is below a carburizing temperature of the material of the lobe pack 22, 24. Each lobe pack 22, 24 is heated at the selected austenitizing temperature for a heating time period. The heating time period is at least long enough to uniformly heat the entire lobe pack 22, 24 to the selected austenitizing temperature. The selected austenitizing temperature may be, for example, at least 800 degrees Celsius, or even at least 850 degrees Celsius. The selected austenitizing temperature, in this example, is high enough to completely form the steel alloy into austenite, but not as high as a carburizing temperature of the particular steel alloy. A carburizing temperature is a temperature at which the steel alloy may absorb carbon liberated from a carbon-bearing material when the steel alloy is heated to a carburizing temperature in the presence of the carbon-bearing material. The carbon-bearing material may include for example, but is not limited to, charcoal or carbon monoxide. The carburizing temperature is above the first temperature at which austenite is fully formed, and in this example, the carburizing temperature is above the selected austenitizing temperate. For example, the carburizing temperature is at least 900 degrees Celsius, and in some examples, the carburizing temperature may be above 950 degrees Celsius.
Referring now to FIG. 4 and with continued reference to FIGS. 1-3, a plot 200 of the heat treatment performed on each lobe pack 22, 24 is illustrated with time in minutes on a horizontal axis 202 and temperature in degrees Celsius on a vertical axis 204. The heat applied to each lobe pack 22, 24 is illustrated as a function of time at the heat trace 206. Thus, each lobe pack 22, 24 is heated to the selected austenitizing temperature 208 for the heating time period 210. The heating time period 210 is long enough to heat the entire lobe pack 22, 24 to the selected austenitizing temperature 208. In this example, the heating time period 210 is at least 45 minutes, but the heating time period 210 could be longer, such as at least one hour long. In other examples, the heating time period 210 could be shorter than 45 minutes. Preferably, the heating time period 210 is no longer than 90 minutes. It is noted that the selected austenitizing temperature 208 is lower than the carburizing temperature 212 of the material, and in some examples, the austenitizing temperature 208 may also be lower than a carbonitriding temperature. Furthermore, neither carburization nor carbonitriding is required, and the method 100 may be performed without carburizing or carbonitriding the outer surface 31 of the lobe packs 22, 24. Therefore, the heating step 104 may take place without placing a carbon source near the outer surface 31 of the lobe pack 22, 24.
Once the heating time period 210 has elapsed, where the entire depth of the lobe pack 22, 24 is heated to the selected austenitizing temperature 208, the method 100 proceeds to a step 106 of quenching the lobe pack 22, 24 to a martempering temperature 214. The martempering temperature 214 is selected to be greater than a martensite start temperature Ms, but within 50 degrees Celsius of the martensite start temperature Ms. In some examples, the martempering temperature 214 is in the range of about 285 to about 330 degrees Celsius, so that the martempering temperature 214 may be selected from any temperature in this range. In some examples, based on using 5150 steel, the martensite start temperature Ms may be in the range of 260 to 295 degrees Celsius. However, it should be appreciated that the martempering temperature 214 will vary for different materials. The selected martempering temperature 214 is preferable about 25 to about 35 degrees Celsius greater than the martensite start temperature Ms. The quenching step 108 is performed directly after the step of heating step 106 without heating the lobe pack above the carburizing temperature 212 between the heating step 106 and the quenching step 108, or even before the heating step 106.
The method 100 then includes a step 108 of holding the lobe pack 22, 24 at the selected martempering temperature 214 for a martempering time period 216. The martempering time period 216 is at least long enough to cool the entire lobe pack 22, 24, including its core, to the martempering temperature 214. Thus, the temperature of lobe pack 22, 24 becomes uniform throughout all sections by holding the temperature applied to the lobe pack 22, 24 at the martempering temperature 214 for the martempering time period 216. In some examples, the martempering time period 216 is at least 25 minutes, or at least 30 minutes. Preferably, the martempering time period 216 is no longer than one hour.
In some examples, quenching and holding the lobe pack 22, 24 at the martempering temperature 214 may be accomplished by submersing the lobe pack 22, 24 in a salt bath that has been heated to the martempering temperature 214. The lobe pack 22, 24 remains submersed in the salt bath until the temperature of the lobe pack 22, 24 is uniform throughout all sections of the lobe pack 22, 24 and substantially equal to the temperature of the salt bath.
Once the temperature of the entire lobe pack 22, 24 has been reduced to the martempering temperature 214, then the lobe pack 22, 24 is cooled to an ambient temperature 218, for example, by removing the lobe pack 22, 24 from the salt bath.
The lobe packs 22, 24 of the sliding camshaft 10 that are manufactured in accordance with the method 100 described above exhibit lower distortion than sliding camshafts conventionally manufactured from high carbon steels, such as AISI 52100 steel. A subsequent hard broaching process may thus be eliminated or lessened because less material will need to be removed to eliminate the distortion. Internal stresses within the lobe packs 22, 24 are also lowered. A majority of the microstructure of each lobe pack 22, 24 is martensite.
It is to be understood that the foregoing is a description of one or more examples. The invention is not limited to the particular example(s) disclosed herein, but rather is defined solely by the claims below. Furthermore, the statements contained in the foregoing description relate to particular examples and are not to be construed as limitations on the scope or on the definition of terms used in the claims, except where a term or phrase is expressly defined above. Various other examples and various changes and modifications to the disclosed example(s) will become apparent to those skilled in the art. All such other embodiments, changes, and modifications are intended to come within the scope of the appended claims.

Claims

What is claimed is:

1. A method of manufacturing a lobe pack for a sliding camshaft of an internal combustion engine, the method comprising:

providing the lobe pack formed from a steel alloy having a carbon content in the range of 0.48 to 0.53 weight percent;

heating the lobe pack to a predetermined austenitizing temperature that is below a carburizing temperature of the steel alloy for a heating time period, the heating time period being at least long enough to heat the entire lobe pack to the predetermined austenitizing temperature;

quenching the lobe pack to a martempering temperature, the martempering temperature being greater than a martensite start temperature and within 50 degrees Celsius of the martensite start temperature; and

holding the lobe pack at the martempering temperature for a martempering time period, the martempering time period being at least long enough to cool the entire lobe pack to the martempering temperature.

2. The method of claim 1, wherein the step of quenching is performed directly after the step of heating without heating the lobe pack above the carburizing temperature of the steel alloy.

3. The method of claim 1, further comprising providing the steel alloy having:

a manganese content in the range of 0.70 to 0.90 weight percent;

a silicon content in the range of 0.15 to 0.35 weight percent; and

a chromium content in the range of 0.70 to 0.90 weight percent.

4. The method of claim 1, further comprising providing the steel alloy as a 5150 steel alloy.

5. The method of claim 4, the martempering temperature being in the range of about 285 to about 330 degrees Celsius.

6. The method of claim 5, wherein the predetermined austenitizing temperature is at least 800 degrees Celsius.

7. The method of claim 6, wherein the carburizing temperature of the steel alloy is at least 900 degrees Celsius.

8. The method of claim 7, wherein the martempering time period is at least 25 minutes.

9. The method of claim 8, wherein the heating time period is at least 45 minutes.

10. The method of claim 9, wherein the steps of quenching the lobe pack to the martempering temperature and holding the lobe pack at the martempering temperature includes submersing the lobe pack within a salt bath.

11. The method of claim 9, further comprising cooling the lobe pack to an ambient temperature after holding the lobe pack at the martempering temperature for the martempering time period.

12. A sliding camshaft for an internal combustion engine, the sliding camshaft comprising:

an inner rod extending along a cam axis;

a lobe pack circumferentially disposed about the inner rod, the lobe pack having an inner side disposed in sliding contact with the inner rod and an outer surface bearing a plurality of cam lobes, the lobe pack being formed of a steel alloy having a carbon content in the range of 0.48 to 0.53 weight percent, wherein a majority of a microstructure of the lobe pack is martensite, the lobe pack having a hardness of at least 56 HRC on the Rockwell hardness scale through a depth of the lobe pack.

13. The sliding camshaft of claim 12, the steel alloy further comprising:

manganese in the range of 0.70 to 0.90 weight percent;

silicon in the range of 0.15 to 0.35 weight percent; and

chromium in the range of 0.70 to 0.90 weight percent.

14. The sliding camshaft of claim 13, further comprising:

phosphorus in an amount not exceeding 0.030 weight percent; and

sulfur in an amount not exceeding 0.040 weight percent.

15. The sliding camshaft of claim 14, wherein the steel alloy is a 5150 steel alloy.

16. The sliding camshaft of claim 15, wherein the lobe pack has a hardness of no greater than 60 HRC on the Rockwell hardness scale through the depth of the lobe pack.

17. The sliding camshaft of claim 16, the lobe pack being hardened via martempering by raising the lobe pack to a predetermined austenitizing temperature that is below a carburizing temperature of the steel alloy and then quenching the lobe pack to a martempering temperature that is above a martensite start temperature.

18. The sliding camshaft of claim 12, wherein the lobe pack is formed by:

quenching the lobe pack to a martempering temperature, the martempering range being greater than a martensite start temperature and within 50 degrees Celsius of the martensite start temperature; and

19. The sliding camshaft of claim 18, wherein the step of quenching is performed directly after the step of heating without heating the lobe pack above the carburizing temperature of the steel alloy, wherein the lobe pack is further formed by cooling the lobe pack to an ambient temperature after holding the lobe pack at the martempering temperature for the martempering time period.

20. The sliding camshaft of claim 19, the martempering temperature being in the range of about 285 to about 330 degrees Celsius, the predetermined austenitizing temperature being at least 800 degrees Celsius, the carburizing temperature of the steel alloy being at least 900 degrees Celsius, the martempering time period being at least 25 minutes, the heating time period being at least 45 minutes.