GB2127716A

GB2127716A - Mechanical system

Info

Publication number: GB2127716A
Application number: GB08224637A
Authority: GB
Inventors: Russell Graham Bent
Original assignee: Cranfield Institute of Technology
Current assignee: Cranfield Institute of Technology
Priority date: 1982-08-27
Filing date: 1982-08-27
Publication date: 1984-04-18
Also published as: FR2532199A1; GB2127716B; FR2532199B1

Abstract

A mechanical system, for use in a centrifuge for example, comprises a double arm 2' fixed to a shaft 4 for rotation in bearings 5 about an axis 3, the double arm 2' carrying at its outer end a unit 1 under test. An arm 7' fixed to the unit 1 extends through a diametrical sliding bearing 9 in the shaft 4 and carries at its other end a counterbalancing mass 8. The average force throughout the length of the double arm 2' is zero or very low, so preventing undesirable extension of the double arm 2' as a result of rotation. <IMAGE>

Description

SPECIFICATION Improvements in or relating to mechanical systems According to the present invention, there is provided a mechanical system comprising a first member extending from an axis of rotation thereof at one side of said axis, bearing means mounting said first member for rotation about said axis, and a second member extending from the radially outer end zone of said first member to that side of said axis opposite to said one side, said second member being such that, on rotation of said first and second members jointly about said axis, said second member applies to said outer end zone a force towards said axis.

In order that the invention may be clearly understood and readily carried into effect, reference will now be made to the accompanying drawings, in which: Figure 1 is a diagrammatic view of a conventional centrifuge, Figure 2 is a view corresponding to Figure 1, but of a centrifuge constituting an example of the present invention, and Figure 3 is a diagrammatic, sectional, axial elevation of the centrifuge of Figure 2.

Referring to Figure 1, a centrifuge is a device for subjecting a unit 1 under test to a range of accelerations. Since, if a body is in motion it will continue in a straight line unless acted upon by an external force, it follows that, with the unit 1 on the end of a rotating arm 2, the unit 1 must be experiencing a force which is causing it to move in a circular path. It can be proved that the force of the unit 1 is constant for a constant speed of rotation equal to F=mrw2, where F is force m is mass r is the radius at which the unit is rotated, and w is the speed of rotation.

The acceleration A seen by the unit 1 is therefore A=rw2 towards the axis of rotation 3.

Since the unit 1 is being accelerated towards the axis of rotation 3, induced by the force F, it follows that the arm 2 must be applying the force and is consequently subjected to a tensile force equal to F. The arm 2 is fixed to a shaft 4 mounted in co-axial bearings 5 and driven by a motor 6.

The bearings 5 would also be subject to the force F, except that this is negated by providing another arm 7 equal in length to the arm 2 and fixed to the shaft 4 diametrically oppositely to the arm 2, with this arm 7 carrying a counter-balancing mass 8 equal to the mass m of the unit 1. The force at the bearings 5 is therefore effectively zero, although each of the arms 2 and 7 still experiences the tensile force F.

Unfortunately, this tensile force F in the arm 2 will cause the arm to elongate until the strain induced by the elongation balances the tensile force F. The increase in the length of the arm 2 causes an increase in the acceleration applied to the unit 1, thereby reducing the accuracy of the centrifuge.

Referring to Figures 2 and 3, the arm 2 is now in the form of a double arm 2', whilst the arm 7 has been increased in length to form an arm 7' extending through a diametrical sliding bearing 9 in the shaft 4 to the unit 1. Thus, since the arm 7' is connected effectively to the outer end of the arm 2', rather than effectively to the inner end of the arm 2, forces similar to those in Figure 1 occur, but the point of application changes to reduce the force in the arm 2' to zero.

Pedantically, this is true only if the arms 2 and 7 are of negligible mass, but, in practice, the counterbalancing mass 8 is such as to cause the average force throughout the length of the arm 2' to be zero. Moreover, it is not necessary actually to have that force reduced to zero, but sufficient to reduce it to a level where the elongation of the arm 2' can be kept below any given tolerances. It will be noted that the force on the bearings 5 is still effectively zero.

The bearing 9 is provided in order to restrain the arm 7' and the mass 8 from a turning movement about the unit 1, as indicated by the dot-dash lines 7" and 8" in Figure 3.

The present invention is applicable to a wide variety of mechanical systems containing rotary arms and where an increase in the lengths of the arms would cause reduced accuracy or reduced efficiency.

Claims (Filed on 23.8.83) 1. A mechanical system comprising a first member extending from an axis of rotation thereof at one side of said axis, bearing means mounting said first member for rotation about said axis, and a second member extending from the radially outer end zone of said first member to that side of said axis opposite to said one side, said second member being such that, on rotation of said first and second members jointly about said axis, said second member applies to said outer end zone a force towards said axis.

2. A system as claimed in claim 1, wherein said second member is guided, in a guide hole which extends substantially perpendicularly to said axis and which rotates with said first and second members, so as to extend through said axis substantially perpendicularly thereto.

3. A system as claimed in claim 1 or 2, and further comprising a third member extending, parallelly and co-extensively with said first member, from said axis at said one side for rotation with said first and second members.

4. A system as claimed in any preceding claim, wherein each of said members is an arm substantially perpendicular to said axis.

5. A system as claimed in any preceding claim, wherein the dimension of said second member perpendicularly to said axis is substantially twice that of said first member.

6. A system as claimed in any preceding claim, wherein a first mass is mounted at the radially outer end zone of said first member and a

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Improvements in or relating to mechanical systems According to the present invention, there is provided a mechanical system comprising a first member extending from an axis of rotation thereof at one side of said axis, bearing means mounting said first member for rotation about said axis, and a second member extending from the radially outer end zone of said first member to that side of said axis opposite to said one side, said second member being such that, on rotation of said first and second members jointly about said axis, said second member applies to said outer end zone a force towards said axis. In order that the invention may be clearly understood and readily carried into effect, reference will now be made to the accompanying drawings, in which: Figure 1 is a diagrammatic view of a conventional centrifuge, Figure 2 is a view corresponding to Figure 1, but of a centrifuge constituting an example of the present invention, and Figure 3 is a diagrammatic, sectional, axial elevation of the centrifuge of Figure 2. Referring to Figure 1, a centrifuge is a device for subjecting a unit 1 under test to a range of accelerations. Since, if a body is in motion it will continue in a straight line unless acted upon by an external force, it follows that, with the unit 1 on the end of a rotating arm 2, the unit 1 must be experiencing a force which is causing it to move in a circular path. It can be proved that the force of the unit 1 is constant for a constant speed of rotation equal to F=mrw2, where F is force m is mass r is the radius at which the unit is rotated, and w is the speed of rotation. The acceleration A seen by the unit 1 is therefore A=rw2 towards the axis of rotation 3. Since the unit 1 is being accelerated towards the axis of rotation 3, induced by the force F, it follows that the arm 2 must be applying the force and is consequently subjected to a tensile force equal to F. The arm 2 is fixed to a shaft 4 mounted in co-axial bearings 5 and driven by a motor 6. The bearings 5 would also be subject to the force F, except that this is negated by providing another arm 7 equal in length to the arm 2 and fixed to the shaft 4 diametrically oppositely to the arm 2, with this arm 7 carrying a counter-balancing mass 8 equal to the mass m of the unit 1. The force at the bearings 5 is therefore effectively zero, although each of the arms 2 and 7 still experiences the tensile force F. Unfortunately, this tensile force F in the arm 2 will cause the arm to elongate until the strain induced by the elongation balances the tensile force F. The increase in the length of the arm 2 causes an increase in the acceleration applied to the unit 1, thereby reducing the accuracy of the centrifuge. Referring to Figures 2 and 3, the arm 2 is now in the form of a double arm 2', whilst the arm 7 has been increased in length to form an arm 7' extending through a diametrical sliding bearing 9 in the shaft 4 to the unit 1. Thus, since the arm 7' is connected effectively to the outer end of the arm 2', rather than effectively to the inner end of the arm 2, forces similar to those in Figure 1 occur, but the point of application changes to reduce the force in the arm 2' to zero. Pedantically, this is true only if the arms 2 and 7 are of negligible mass, but, in practice, the counterbalancing mass 8 is such as to cause the average force throughout the length of the arm 2' to be zero. Moreover, it is not necessary actually to have that force reduced to zero, but sufficient to reduce it to a level where the elongation of the arm 2' can be kept below any given tolerances. It will be noted that the force on the bearings 5 is still effectively zero. The bearing 9 is provided in order to restrain the arm 7' and the mass 8 from a turning movement about the unit 1, as indicated by the dot-dash lines 7" and 8" in Figure 3. The present invention is applicable to a wide variety of mechanical systems containing rotary arms and where an increase in the lengths of the arms would cause reduced accuracy or reduced efficiency. Claims (Filed on 23.8.83)

1. A mechanical system comprising a first member extending from an axis of rotation thereof at one side of said axis, bearing means mounting said first member for rotation about said axis, and a second member extending from the radially outer end zone of said first member to that side of said axis opposite to said one side, said second member being such that, on rotation of said first and second members jointly about said axis, said second member applies to said outer end zone a force towards said axis.

6. A system as claimed in any preceding claim, wherein a first mass is mounted at the radially outer end zone of said first member and a counterbalancing mass is mounted at the opposite end zone of said second member.

7. A system as claimed in claim 6 and in a centrifuge, said first mass being a unit under test.

8. A mechanical system, substantially as hereinbefore described with reference to Figs. 2 and 3 of the accompanying drawings.