GB2514216A

GB2514216A - Drive shafts

Info

Publication number: GB2514216A
Application number: GB1403881.4A
Authority: GB
Inventors: Stuart Cartmell
Original assignee: Gilbert Gilkes & Gordon Ltd
Current assignee: Gilbert Gilkes & Gordon Ltd
Priority date: 2013-04-02
Filing date: 2014-03-05
Publication date: 2014-11-19
Also published as: GB201403881D0; GB201305862D0

Abstract

A composite drive shaft for a pump 1 for handling a corrosive and/or abrasive fluid and a method of manufacturing it are provided. The composite shaft comprises a first part 12 of a first metal / alloy resistant to the corrosive/abrasive fluid, connected to the pumping elements 3 of the pump 1. When in use, the first part 12 is actually or potentially in contact with the corrosive / abrasive fluid inside the pump 1. The first part 12 extends outside the pump body through a rotary seal 5. The shaft further comprises a second part 9 of a second cheaper metal/alloy, axially aligned with and abutting the first part 12 and connected to the extended length of the first part 12 by a butt friction weld. The second part 9 of the composite shaft is connectable to the power source for driving the pump 1. Both first and second metals/alloys have adequate mechanical properties to operate the pump reliably for a full working life.

Description

DRIVE SHAFTS

This specification relates to drive shafts for pumps and the like, where at least a part of the drive shaft comes into contact with a potentially corrosive, abrasive or otherwise injurious substance(s). It is particularly, but not exclusively, relevant to engine draw seawater cooling pumps.

Any designer naturally wants to create the best quality product that he I she can but they io are always restrained by the mechanical limitations of the materials, their cost, manufacturing costs and often other factors, such as environmental considerations, and permitting easy recycling of the product at the end of its working life, etc. The considerations of different markets and price brackets usually lead to a range of products in any particular field. Thus, there are very expensive, luxurious, high quality cars at the top end of the market and a variety of less expensive cars, with lower levels of quality and fewer luxury fittings, for those with smaller budgets.

Within each price range, the various components are designed to fit together as a whole.

For example, an item with a design life of 200 years is not normally connected to other components expected to last only, say, 20 or fewer years. (This does not apply to items which wear in their normal lives, e.g. brake linings.) However, there are exceptions where the health or safety of the users could be endangered by the failure of a single component or a whole system. Thus, for example, the reliability of the braking systems in cars should* be greater than that of, say, the engine; this is why cars have two independent braking systems (a hydraulic footbrake and a mechanical hand brake) but only one engine.

This principle is similar with other systems that could have an effect far beyond failure of a component in that particular system. For example, a ship at sea is affected by the vagaries of winds, tides and currents and relies on its engine to keep it out of danger, so if, for any reason, the engine failed, the ship and its passengers could be in mortal danger. Systems which could cause sudden and total engine failure include the fuel supply and cooling means. Thus, in the case of a cooling system using potentially corrosive seawater as a medium and where abrasive particles are often drawn in while in port or shallow water, it is not only justifiable, but essential, to marry a highly reliable pump, with, say, a design life of 200 years, to an engine with an expected life of, possibly, only a tenth of this.

As with all designs, cost is always a major consideration and the skilled designer must use highly innovative skills to create an acceptably reliable design at a price which the customer can afford.

According to the invention, there is provided a composite drive shaft for a pump for handling a corrosive and / or abrasive fluid comprising:-I) a first shaft of a first metal / alloy connectable to the pumping elements of the pump and which, when in use, is actually or potentially in contact with the corrosive I abrasive fluid in the internal environment inside the pump; ii) a seal around the first shaft, which when in use, separates the corrosive I abrasive fluid inside the pump from the external environment outside the pump and is located such that a length of the first shaft projects through the seal out of the pump into the external environment; iii) a second shaft of a second metal / alloy, axially aligned with and abutting the first shaft and connected to the projecting length of the first shaft by a butt friction weld said second shaft also being connectable to the power source for driving the pump; characterised in that the first metal I alloy is of a corrosion and for abrasion resistant material well able to withstand being in contact with the fluid and that the second metal I alloy is a cheaper readily available material having different but adequate mechanical properties to those of the first metal / alloy and further characterised in that there is only a minimal axial length of the first metal I alloy shaft between the seal and the friction weld thus providing the required mechanical strength overall and corrosion and I or abrasion resistance inside the body of the pump yet minimising the total cost of the composite driving shaft and also further characterised in that the friction weld, after completion, requires no subsequent machining but only basic fettling.

According to a first variation of the composite drive shaft of the invention, the corrosive fluid is seawater.

According to a second variation of the composite drive shaft of the invention, the fluid contains an abrasive substance.

According to a third variation of the composite drive shaft of the invention, the first metal I alloy is from any corrosion resistant and I or abrasion resistant material.

According to a fourth variation of the composite drive shaft of the invention, the first metal I alloy is from an austenitic I duplex stainless steel group of alloys.

According to a fifth variation of the composite drive shaft of the invention, the first metal I alloy is from 316 or 318 stainless steel.

According to a sixth variation of the composite drive shaft of the invention, the first metal I alloy is from a copper I nickel alloy.

According to a seventh variation of the composite drive shaft of the invention, the seal is a conventional item, bearing on the circumference of the shaft but allowing the shaft to rotate while maintaining the sealing.

According to an eighth variation of the composite drive shaft of the invention, the second metal I alloy is a high quality carbon steel or an equivalent material having different but adequate mechanical properties to those of the first metal / alloy.

According to a ninth variation of the composite drive shaft of the invention, the friction welding is undertaken in an apparatus similar to a lathe so that the first and second shafts are joined by a butt weld and aligned about a common axis of rotation.

According to a tenth variation of the composite drive shaft of the invention, the friction welding apparatus includes means to remove the friction welding debris after, or as part of, the friction welding process and whilst still in the welding apparatus.

According to an eleventh variation of the composite drive shaft of the invention, the minimal axial length of the first shaft between the edge of the line of action of the seal and the friction weld is between 0.2 and 2.0 times the diameter of the shaft at that point.

According to a twelfth variation of the composite drive shaft of the invention, the shaft is machined to accept filling of the pumping elements and I or bearing elements and I or connection to the driving member and I or in way of the seal.

According to a thirteenth variation of the composite drive shaft of the invention, the fettling Includes wire brushing, use of abrasives and I or use of a file.

According to the invention, there is provided a method of making a composite shaft for a pump for handling a corrosive and / or abrasive fluid comprising the steps of:-i) providing a first shaft of a first metal I alloy connectable to the pumping elements of the pump and which, when in use, is actually or potentially in contact with the S corrosive I abrasive fluid in the internal environment inside the pump; ii) providing a seal around the first shaft which, when in use, separates the corrosive / abrasive fluid inside the pump from the external environment outside the pump and is located such that a length of the first shaft projects through the seal out of the pump into the external environment; iii) providing a second shaft of a second metal I alloy, axially aligned with and abutting the first shaft and connected to the projecting length of the first shaft by a butt friction weld said second shaft also being connectable to the power source for driving the pump; characterised in that the first metal I alloy is of a corrosion and I or abrasion resistant material well able to withstand being in contact with the fluid and that the second metal I alloy is a cheaper readily available material having different but adequate mechanical properties to those of the first metal / alloy and further characterised in that there is only a minimal axial length of the first metal I alloy shaft between the seal and the friction weld thus providing the required mechanical strength overall and corrosion and I or abrasion resistance inside the body of the pump yet minimising the total cost of the composite driving shaft and also further characterised in that the friction weld, after completion, requires no subsequent machining but only basic fettling.

According to a first variation of the method of making a composite shaft of the invention, the friction welding debris is removed after, or as part of 1 the friction welding process and whilst still in the welding apparatus.

In a preferred application of the invention, a composite shaft for mounting and driving the elements of a pump for handling a corrosive and / or abrasive fluid is disclosed. The shaft is a composite unit having a first part made of a corrosion and / or abrasion resistant metal or alloy, situated inside the pump housing where it will actually, or potentially, be in contact with the corrosive I abrasive fluid. The first part of the shaft extends axially out of the pump housing via a rotary seal, to keep the corrosive I abrasive fluid inside the pump body. The first part of the shaft terminates in a stub axle just beyond of the line of action of the seal, i.e. extending outside the pump body.

Fast with the stub axle, via an axial friction butt weld, is the second part of the composite shaft made of a cheaper metal or alloy having different, but adequate, mechanical properties to those of the corrosion resistant metal I alloy. The first part of the composite shaft is adapted to carry the pumping elements and mate with the seal and the second part is adapted to carry bearings and connect to the driving member.

The key aspect of the invention is to provide a full-length, full-strength shaft for the pump but to minimise the cost by reducing the axial length of the expensive corrosion I abrasion resistant part to the absolute minimum and also optimising the bar stock to have both the required mechanical strength and corrosion resistance properties. Friction welding provides a full penetration weld much more quickly and cheaply than by conventional MIG or TIG processes. During the frictional welding process, weld debris is removed to leave an essentially smooth circumference.

is For a clearer understanding of the invention and to show how it may be carried into effect, reference will now be made, by way of example only, to the accompanying drawings in which: -Figure 1 is a general arrangement of an engine cooling pump according to the invention, showing the seawater inlet 2 and drive shaft 9; Figure 2 is a diagrammatic representation of a conventional butt weld between two

shafts (Prior Art);

Figure 3 is a diagrammatic representation of the preparation for a friction shaft butt welding process according to the invention; Figure 4 is a is a diagrammatic representation of the commencement of a friction shaft butt welding process according to the invention; Figure 5 is a diagrammatic representation of the friction shaft butt welding according to the invention part way through the process; and Figure 6 is a diagrammatic representation of the completed friction shaft butt weld according to the invention.

In the following description, the same reference numeral is used for identical parts in different Figures or for different parts fulfilling identical functions.

Figure 1 shows a general arrangement of a seawater pump Ito cool a ships engine (not shown). Seawater enters via duct 2, is pumped by impeller 3 and passes through volute 4 to cool the engine. Impeller 3 is fast with shaft 12 and rotated 10 via connecting shaft 9.

Shaft 9 rotates in bearings 7 and 8; bearing 7 is shown as a double row to carry the encastré load of pump 1. Seal 5 keeps the seawater inside pump 1 and seal 6 retains the lubrication in bearings 7 and 6. Lines 5A show the limits of seal 5 and its contact with shaft 12.

As shown, shaft 9 and 12 is a continuous member. However, part 12 to the right of seal 5 is in contact with seawater and, as stated previously, this is corrosive (even at low temperatures). It may also contain abrasive materials (sand, grit, etc.), impurities (oil, chemicals, etc.) and marine growths, etc. As stated previously, the engine cooling is a Level One System, i.e. one where failure would potentially endanger the ship and the lives of those on board. Thus, shaft 12 must be of a material resistant to seawater corrosion -this means an appropriate grade of stainless steel, such as martensitic I ferritic duplex steels. Grade 316 stainless steel is preferred but grade 316 may be used for some applications; these are not standard grades of austenitic stainless steel and are consequently very expensive but, unlike the standard grades, do not suffer from stress corrosion cracking.

Copper I nickel alloys such as the Inconel® or Monel® ranges of alloys are also suitable for this application.

Stress corrosion cracking might not seem to be important to the casual observer thinking of a ship sailing sedately across an ocean at a constant speed but the majority of ships, e.g. ferries, have a much more varied life -stopping, starting, manoeuvring in the first port, making the crossing and repeating everything in the other port. Additionally, engine cooling pumps are normally bolted fast to the engine block via an auxiliary housing (not shown), with the drive being taken directly from the engine output gear train. Despite the smoothing effects of a flywheel, whenever a piston has a firing stroke, an impulse is passed through the gear train causing an angular acceleration to be transmitted directly to pump shaft 9 in the form of fatigue induced torsional vibrations, which are superimposed on the essentially smooth rotation 10.

Thus, accommodating a fluctuating stress loading due to torque is an important factor in the design of shaft 9, 12, particularly part 9, and, in the case of a single piece shaft, would dictate the use of an extremely expensive material, such as 17-4PH stainless steel (which this disclosure seeks to replace), both to withstand the high transmission loads and provide corrosion resistance for the length (12) in contact with the seawater.

However, it is more than possible to carry the drive loads, in an area unaffected by corrosive I abrasive media, with a medium carbon steel that has been heat treated and, by friction welding this material to grade 318 stainless steel at plane 11, provide optimisation of the raw material to overcome the two main technical hurdles, i.e. corrosion / abrasion resistance and mechanical I endurance strength, and simultaneously minimise the cost.

Generally, welding carbon steel (MS) 9Ato stainless steel (85) 12A is not recommended but it can be done (Fig. 2), e.g. with a full penetration weld under inert gas (MIG or TIG) between weld preparations 13. However, this is a costly, skilled operation which leaves a large heat affected zone (HAZ) 14 and the net cost saving over the equivalent full SS shaft is minimal.

To overcome these two unsatisfactory options friction welding (or direct drive friction welding) is employed. Figs.3-6 show the process in diagrammatic form. A carbon steel billet 9B is placed in the stationary (i.e. non-rotatable) stock 15 of a lathe-type apparatus (not shown) and a stainless steel billet 1 2B is placed in the rotating chuck (not shown), (or vice versa). Billet 12B is rotated 18, whilst billet 9B is moved axially 19 until contact between the two end faces 16 and 17 is made, without the addition of any filler material.

After contact, the surfaces 16 and 17 experience relative, rotary motion, resulting in friction, which leads to localised heating of the two faces. Once sufficient heat has been generated, e.g. circa 1055°C, relative motion 18 is ceased and a large axial force 20, e.g. 600kN, is applied to the work piece forging the two billets together along plane 21, resulting in the final joined work piece.

During the heat generation, friction phase, metal is displaced outward leaving a flash 22 at the outside diameter of the two billets. This flash 22 is removed 24 using a cutting tool 23, similar to that used in standard turning operations and is done while the metal is still hot and soft. The shaft is then allowed to cool in air. Subsequently heat treatment, consisting of a stress relieving anneal, is carried out on the shaft to remove residual stresses and soften the HAZ 25 to facilitate downstream machining operations.

This method of manufacture is readily reproducible, takes less than 30 seconds, produces a small HAZ 25 and at a fraction of the cost of a full penetration weld (Fig. 2) or of a single piece of 318 stainless steel shaft.

On completion of the friction welding and any de-stressing or hardening operations, the composite shaft 9B, 12B is machined using any standard machining methods, e.g. providing keyways to accommodate the pumping elements 3 and / or connection to the driving member (not shown) and I or with lands to accept the bearings 7, 8 and / or in way of seals 5 or 6. It will be noted that no machining is required in way of the friction weld. All that is required would be basic fettling, e.g. wire brushing, use of abrasives or possibly of a file. There would be machining in way 5A of seal 5 but this is separated from friction weld 11 by length 29 of shaft 12.

The pump may now be assembled, tested and set to work in the confident knowledge that, with proper maintenance, it will function reliably for at least its design lifetime.

The invention teaches the design logic under which an apparently insurmountable problem, i.e. resolving the conflicting requirements of strength, corrosion / abrasion resistance and cost, can be solved in an acceptable manner to give a high quality, fully funétional, cost-effective and reliable solution. It minimises the length of the expensive SS shaft, without, in any way, compromising the design -in fact, friction welding allows the conflicting requirements to be optimised and is quite possibly a better engineering solution than the full weld (Fig. 2) or a shaft made from a single billet of stainless steel.

To appreciate further the benefit of the invention, the full length 26 of the 318 SS shaft 12 is indicated (Fig. 1) as is the length 27 of the carbon steel shaft 9. In a typical application, the ratio on the lengths 26:27 is 1:1.7, i.e. the invention saves 63% of the length of the composite shaft 9/12 and so effectively 63% of the cost of a full length 17-4PH SS shaft.

Defining the length 29 between the extreme left hand line of action 5A of seal 5 and weld 11 is not straightforward. Fig. I shows a design of pump I and drive shaft 9 which is produced in essentially geometrically similar forms with pumping capacities ranging from about 12m3/hr up to 150m3/hr. In these two cases,, the shaft diameter 28 in way of seal 5 and weld 11 would be about 25mm and 55mm respectively. Bearing in mind, practical manufacturing parameters and the robust' operating conditions in a ship's engine room in which pump 1 would have to operate, clearance 11 for the smaller pump could be as little as 5mm and perhaps as much as 100mm for the larger pump. This is a range from 0.2 to nearly 2.0 times the shaft diameter 28 and is indicative of what is a manufacturing practicality and desirable to locate in the actual operational environment.

Other examples of the principle, disclosed herein, will be apparent to the person skilled in the ad, all falling within the scope of the invention.

Claims

Claims:- 1. A composite drive shaft for a pump for handling a corrosive and I or abrasive fluid comprising:-I) a first shaft of a first metal I alloy connectable to the pumping elements of the pump and which, when in use, is actually or potentially in contact with the corrosive I abrasive fluid in the internal environment inside the pump; ii) a seal around the first shaft, which when in use, separates the corrosive I abrasive fluid inside the pump from the external environment outside the pump and is located such that a length of the first shaft projects through the seal out of the pump into the external environment; iii) a second shaft of a second metal I alloy, axially aligned with and abutting the first shaft and connected to the projecting length of the first shaft by a butt friction weld said second shaft also being connectable to the power source for driving the pump; characterised in that the first metal / alloy is of a corrosion and I or abrasion resistant material well able to withstand being in contact with the fluid and that the second metal I alloy is a cheaper readily available material having different but adequate mechanical properties to those of the first metal I alloy and further characterised in that there is only a minimal axial length of the first metal / alloy shaft between the seal and the friction weld thus providing the required mechanical strength overall and corrosion and / or abrasion resistance inside the body of the pump yet minimising the total cost of the composite driving shaft and also further characterised in that the friction weld, after completion, requires no subsequent machining but only basic fettling.
2. A composite drive shaft, as claimed in claim 1, wherein the corrosive fluid is seawater.
3. A composite drive shaft, as claimed in claims 1 and I or 2, wherein the fluid contains an abrasive substance.
4. A composite drive shaft, as claimed in any preceding claim, wherein the first metal I alloy is from any corrosion resistant and / or abrasion resistant material.
5. A composite drive shaft, as claimed in claim 4, wherein the first metal / alloy is from an austenitic / duplex stainless steel group of alloys.
6. A composite drive shaft, as claimed in claim 5, wherein the first metal / alloy is from 316 or 318 stainless steel.
7. A composite drive shaft, as claimed in claim 4, wherein the first metal I alloy is from a copper I nickel alloy.
8. A composite drive shaft, as claimed in any preceding claim, wherein the seal is a conventional item, bearing on the circumference of the shaft but allowing the shaft to rotate while maintaining the sealing.
9. A composite drive shaft, as claimed in any preceding claim, wherein the second metal I alloy is a high quality carbon steel or an equivalent material having different but adequate mechanical properties to those of the first metal I alloy.
10. A composite drive shaft, as claimed in any preceding claim, wherein the friction welding is undertaken in an apparatus similar to a lathe so that the first and second shafts are joined by a bull weld and aligned about a common axis of rotation.
II. A composite drive shaft, as claimed in claim 10, wherein the friction welding apparatus includes means to remove the friction welding debris after, or as part of, the friction welding process and whilst still in the welding apparatus.
12. A composite drive shaft, as claimed in any preceding claim, wherein the minimal axial length of the first shaft between the edge of the line of action of the seal and the friction weld is between 0.2 and 2.0 times the diameter of the shaft at that point.
13. A composite drive shaft, as claimed in any preceding claim, wherein the shaft is machined to accept fitting of the pumping elements and I or bearing elements and I or connection to the driving member and I or in way of the seal.
14. A composite drive shaft of the invention, as claimed in any preceding claim, wherein the fettling includes wire brushing, use of abrasives and I or use of a file.
15. A method of making a composite drive shaft for a pump for handling a corrosive and / or abrasive fluid comprising the steps of:-i) providing a first shaft of a first metal I alloy connectable to the pumping elements of the pump and which, when in use, is actually or potentially in contact with the corrosive / abrasive fluid in the internal environment inside the pump; ii) providing a seal around the first shaft which, when in use, separates the corrosive I abrasive fluid inside the pump from the external environment outside the pump and is located such that a length of the first shaft projects through the seal out of the pump into the external environment; iii). providing a second shaft of a second metal I alloy, axially aligned with and abutting the first shaft and connected to the projecting length of the first shaft by a butt friction weld said second shaft also being connectable to the power source for driving the pump; characterised in that the first metal / alloy is of a corrosion and I or abrasion resistant material well able to withstand being in contact with the fluid and that the second metal / alloy is a cheaper readily available material having different but adequate mechanical properties to those of the first metal / alloy and further characterised in that there is only a minimal axial length of the first metal / alloy shaft between the seal and the friction weld thus providing the required mechanical strength overall and corrosion and I or abrasion resistance inside the body of the pump yet minimising the total cost of the composite driving shaft and also further characterised in that the friction weld, after completion, requires no subsequent machining but only basic fettling.
16. A method of making a composite drive shaft for a pump, as claimed in claim 15, wherein the friction welding debris is removed after, or as part of, the friction welding process and whilst still in the welding apparatus.
17. Apparatus and method of making a composite drive shaft for a pump as described in and by the above description with reference to Figs. 1 and 3-6 of the accompanying drawings.