GB2150063A

GB2150063A - Assembly of metal part and separate element

Info

Publication number: GB2150063A
Application number: GB08428723A
Authority: GB
Inventors: Jean Bret
Original assignee: Guinard Centrifugation
Current assignee: Guinard Centrifugation
Priority date: 1983-11-23
Filing date: 1984-11-14
Publication date: 1985-06-26
Also published as: DD229062A5; JPS60132110A; IT8412638A1; FR2555085A1; GB8428723D0; IT1180488B; IT8412638A0; FR2555085B1

Abstract

A protective plate 1 is secured to the metal thread 2 of a conveyor or other screw by a hole 4 being provided in the plate 1, and a pin blank being located in the hole 4 and electrically welded to the thread 2 and the plate 1 while being compressed between the welding electrodes E and F to form a pin 3 with a head fitting a widened outer end of the hole 4. The plate 1 is made of sintered tungsten carbide. The pin blank does not completely fill the hole 4 before it is compressed. <IMAGE>

Description

SPECIFICATION Assembly of metal part and separate ele ment The present invention relates to assemblies constituted by a metal part and an element which is added thereto, as well as to methods of making these assemblies. It applies especially to the securing of a protective element to a part in which it is difficult to effect piercing or tapping, because for example, one of its faces is inaccessible, and, most especially, to the securing of an element which is a less good conductor of electricity than a metal and is resistant to abrasion, such as a tungsten carbide plate, to the thread of a screw such as the conveyor screw of a centrifuge, a decanter, a drying machine or other separating machine.

The present-day technique consists in protecting the metal surfaces subjected to abrasion by hard coatings, such as: -electrolytically deposited chromium (thickness 0.2 to 0.3 mm) or alloys thereof with cobalt and tungsten, -ceramic deposited directly on the metal to be protected or added as small screwed on and glued plates, -tungsten carbide in the form of powder or rod, deposited by fusion or disposed in the form of sintered plates by brazing.

This latter coating based on sintered plates fixed by brazing gives the best performance, but it has the disadvantage of not resisting an acid medium (pH less than 4.5), because of the medicore resistance of the silver brazant.

Moreover, many precautions must be taken to avoid thermal shocks during the cooling of the plates, which can cause fatigue resulting in cracks or even bursting.

There are coatings with sintered plates secured mechanically by a dovetail device on a milled intermediate support. This support is then added to the helical thread. This device withstands an acid medium, but is relatively expensive, if only because of it being necessary to make it in two stages and the machining which it entails.

The assembly according to the invention has good resistance to acid media and is inexpensive to make because of the simplicity of the coupling operations which are carried out without danger of cracking and bursting and without the need for complicated structural elements, and the small number of parts brought into operation.

The subject of the invention is an assembly constituted by a metal part, an element which is added thereto and which is pierced by a hole, and a pin engaged in the hole, characterised in that the metal pin is retained by forging to the side wall of the hole, which is pierced in the element only, and one of the ends of the pin is welded to the part.

The method of making this assembly consists in placing a part and an element pierced by a hole in contact, engaging a metal pin blank in the hole, applying one welding electrode to the part at the place opposite the nearest end of the blank and a complementary welding electrode to the other end of the blank, and, while pressing the part and the blank towards one another, welding the one end of the blank to the part and transforming the blank into a pin.

The welding and the forging are thus carried out in one and the same very reliable operation. There is no longer any silver brazant which is sensitive to acids.

The securing of the element to the part is further improved when the hole is widened out at the side opposite from the part. The widened out portions, in addition to the retention by forging to the sidewalls of the holes, resist any separation of the part and the element.

According to a particularly useful variant for skewed parts such as screw threads, a third hole opening into a recess at the side by the part is pierced in the element between the two other holes and is filled in with a mass of adhesive. This third hole, which is smaller than the two others, then allows injection of an adhesive agent, such as an expoxy glue, to fill up the gap which can be produced after welding of the pins and to obtain a plane seating on a portion of the thread.

Preferably the element has a positioning boss produced on the face by the part.

It is recommended that the electrical resistance of the metal constituting the welding electrode remote from the part be greater than that of the metal constituting the welding electrode close to the part. On account of this, the pin blank flows more and faster on forging of the remote side of the part where the widened out part of the hole is provided.

It is desirable to limit the constraint that the pin puts on the side wall of the hole in order to avert any danger of bursting. To this end, it is advisable for the blank not to have a cross section similar to that of the hole and/or for the blank to be tubular and/or to be uneven.

The blank can for example be threaded, knurled or prismatic in a round hole. Uneven forms ensure better welding by concentrating the passage of the current in the body of the blank and by isolating the latter better from the element.

In the attached drawing, given solely by way of example: Figure 1 is an elevation of a portion of the thread of a conveyor screw of a centrifuge, on which, according to the invention, a sintered plate of tungsten carbide is added, Figure 2 is a sectional view of this portion, Figure 3 is a view of a pin blank which can be used according to the invention, and Figure 4 is a sectional view of another variant of pin blank.

A sintered tungsten carbide plate 1, serving as an element protecting against wear by abrasion, is added to a skewed portion 2 of steel screw thread by two pins 3 forged into holes 4 pierced in the plate 1.

The plate 1 is automatically positioned by bosses 5 which are produced on the face of the plate 1 by the thread 2 and become supported on the edge of the latter.

Each hole 4 comprises a central part of circular cross section opening, at the side by the thread 2, into a recess 6 and, at the opposite side, into a widened out part 7.

A hole 8, of much smaller diameter than the holes 4, is pierced in the plate 1 between the holes 4 and opens into the recess 6. This hole 8 and the recess 6 are filled up with Araldite (epoxy glue).

A metal pin 3 is retained in each hole 4 by forging and by complementarity of form with the widened out part 7. The pin 3 is spot welded to the thread 2 by the end closest to the latter. This end is of stainless steel.

To secure the plate 1 to the thread 2, the following operation is carried out: The plate 1 is put in position on the thread 2 by means of the bosses. Pin blanks of 5 mm diameter and 6.5 mm length are engaged in the holes 4 of 5.2 mm diameter until their end touches the thread 2. This end is welded to the thread 2 by putting a copper electrode E of a spot welding machine in contact with the place on the thread 2 opposite this end and the other complementary electrode of copper and tungsten of this machine in contact with the end of the blank furthest from the thread 2. Welding (current strength 600A) is carried out while pressing the blank (240 daN) and the thread 2 towards one another so that the material of the blank flows radially and the latter is transformed into a pin 3 locked into the hole 4 by the side face. An adhesive is injected into the hole 8 and the recess 6.

Fig. 3 illustrates a pin blank which is threaded, after welding and forging of the head.

Fig. 4 illustrates a tubular prismatic blank.

Claims

1. Assembly constituted by a metal part (2), a separate element (1), which is pierced by a hole (4), and a pin (3) engaged in the hole (4), characterised in that the metal pin (3) is retained by forging to the side wall of the hole (4), which is pierced in the element (1) only, and one of the ends of the pin (3) is welded to the part (2).

2. Assembly according to claim 1, characterised in that the hole (4) and the pin (3) are widened out at the opposite side from the part (2) after forging.

3. Assembly according to claim 1 or 2, characterised in that the element (1) is pierced by several holes (4) where pins (3) are engaged.

4. Assembly according to claim 3, characterised in that a supplementary hole (8), opening into a recess at the side by the part, pierces the element (1) between the holes (4) and is filled up with an adhesive mass.

5. Assembly according to one of the preceding claims, characterised in that the element has a positioning boss (5) produced on the face by the part (2).

6. Assembly according to one of the preceding claims, characterised in that the part is the thread of a screw and the element is a sintered plate with a basis of tungsten carbide.

7. Method of making an assembly according to one of the claims 1 to 6, characterised in that it consists in putting a part and an element pierced by a hole in contact, engaging a metal pin blank in the hole, applying a welding electrode to the part at the place opposite the nearest end of the blank and a complementary welding electrode to the other end of the blank, and, while pressing the part and the blank towards one another, welding the one end of the blank to the part and transforming the blank into a pin by forging.

8. Method according to claim 7, characterised in that the blank does not have a cross section similar to that of the hole.

9. Method according to claim 7 or 8, characterised in that the blank is tubular and/or is uneven.