Compensation of compressor rods deformations using superbolts.

Eftimie, Lucian ; Tierean, Mircea ; Baltes, Liana 等

Abstract: The paper presents the using of superbolts to compensate the compressor rod manufacturing run-outs. It describes the technical aspects of the superbolt design and the results obtained for a practical application. Using F.E.A. it is possible to establish the variation of radial deformation of the rod produced by the different clamping force on the jackbolts.

Key words: compressor, superbolt, FEA, piston.

1. INTRODUCTION

The reciprocating compressors increases the pressure of the fluid using the longitudinal movement of the piston placed on the rod, driven by a crank mechanism. Many difficulties arise due to high rod lengths, especially during machining and final heat treatment. Thus the rod experiences significant deformations such as longitudinal alignment and radial run-out. This inconvenience can be adjusted using a special fixing system of the piston on the rod.

2. SUPERBOLT, THE MULTI-JACKBOLT TENSIONER

The superbolt is a direct replacement for hex nuts, covered nuts, bolts, etc. They mount onto existing bolt or stud and provide a better way to bolt up the joint. They consist of three components:

* a hardened washer which protects the equipment and provides a hardened, flat surface for the jackbolts to "push" against;

* a nut body which spins on the existing bolt or stud and seats against the washer hand tight (superbolt nut bodies are typically round);

* jackbolts, which thread through the nut body and are used to tighten the joint in pure tension. The jackbolts are tightened using ordinary hand tools. Advantages of the superbolts are:

* tremendous clamping force available;

* low torque requirement--only hand tools needed;

* safe method for installation personnel;

* reduces installation time over common methods;

* tightens stud/bolts in pure tension:

** no thread galling,

** no need for tight tolerance threads, which can seize,

** studs will not seize into housing;

* tensioner flexibility adds elasticity to studs/bolts, which creates a better bolted joint;

* fits in restricted areas;

* provides even tension from stud to stud;

* accurate within 10% of specified bolt tension;

* easy to disassemble.

Another important facility of the superbolt is the possibility to apply different axial forces on the jackbolts, which means the introduction of bending moment on the rod. Adequate dimensioning of this bending moment can correct the imperfections introduced in the rod during the technological process. The subject of present study is to determine the magnitude of the clamping forces on the jackbolts able to compensate the rod's deformation.

[FIGURE 1 OMITTED]

3. THE ROD-PISTON ASSEMBLY

For low differences in diameters between piston and rod, a complete piston is used, divided in three parts (figure 1). The components are assembled using a superbolt. The advantage of this solution consists in avoiding the requirement for a special heavy tool and a high fixing torque. Figure 1 presents the full piston assembly. The piston is positioned into place with a small longitudinal key. To improve the ease of maintenance, the piston is divided in three parts.

For big diameter differences between piston and rod, a piston with cavities is used to reduce the weight, also divided in three parts (figure 2). Fluid leaks into the piston cavities are avoided by sealing them with gaskets. Leaks would increase the effective dead volume of the cylinder and change the compressor operating cycle.

4. COMPENSATION OF DEFORMATIONS USING SUPERBOLT

To study the deformations (figure 3) introduced by the different clamping forces of the jackbolts in the piston-rod assembly, finite elements analysis was used (Zienkiewicz & Taylor 2000). The 3D model of the rod was done using Autodesk Inventor. For FEA we have used Cosmos DesignStar 4.0 FEA package (S.R.A.C., 1998).

The mesh for the FEA contained 11708 (type of elements) elements. The rod in this study used a superbolt with 8 jackbolts. One of the jackbolts was loaded with a constant axial force, while the remaining jackbolts were loaded with constantly decreasing axial forces. The opposite end of the rod is considered fixed.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

The magnitude of the axial forces was chosen not to affect the contact between piston, rod and the hardened washer. Figure 4 shows the static strains of the rod during the superbolt clamping.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

The Von Mises stress map is presented in figure 6. Figures 5 and 7 present the variations of the radial displacements in the piston end section and Von Mises stresses in the fixing section produced by different clamping force on the jackbolts.

The results of the present study were successfully applied at the Braflex Ltd. Company Brasov, Romania, where 0.17 mm radial run-out needed to be compensated.

5. CONCLUSION

[FIGURE 7 OMITTED]

Using the superbolt clamping system it is possible to correct significant manufacturing errors in compressor rod production.

Considering the measured radial run-out on the rod, and using the curvature presented in the figure 5, one it must apply different clamping forces on the jackbolts to solve the problem.

6. REFERENCES

Zienkiewicz, O.C. & Taylor, R.L. (2000). The Finite Element Method, Butterworth-Heinemann, ISBN 0 7506 5049 4, Oxford

S.R.A.C. (1998). Cosmos/M 2.0 Manuals. Los Angeles, CA www.superbolt.com