Login| Sign Up| Help| Contact|

Patent Searching and Data


Title:
TENSION BOLT
Document Type and Number:
WIPO Patent Application WO/2009/068737
Kind Code:
A1
Abstract:
A fastening screw (1) comprising two end sections (2), at least one of which is provided with threads, and a shank (3) between the end sections (2). The cross-sectional shape of the shank (3) deviates from a circle.

Inventors:
LILJENFELDT, Gösta (Myrvägen 137 B, Solf, FI-65450, FI)
SÄRS, Harry (Skolhusgatan 70 A 14, Vasa, FI-65100, FI)
NYNÄS, Håkan (Nyängsvägen 3 B 2, Vasa, FI-65280, FI)
SUNDSTEN, Magnus (Söderbyvägen 90, FI- Korsholm, 65610, FI)
Application Number:
FI2008/050678
Publication Date:
June 04, 2009
Filing Date:
November 24, 2008
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
WÄRTSILÄ FINLAND OY (Tarhaajantie 2, Vaasa, FI-65380, FI)
LILJENFELDT, Gösta (Myrvägen 137 B, Solf, FI-65450, FI)
SÄRS, Harry (Skolhusgatan 70 A 14, Vasa, FI-65100, FI)
NYNÄS, Håkan (Nyängsvägen 3 B 2, Vasa, FI-65280, FI)
SUNDSTEN, Magnus (Söderbyvägen 90, FI- Korsholm, 65610, FI)
International Classes:
F16B35/04; F16B31/06
Foreign References:
US4338054A
DE661711C
DE624729C
DE916608C
US2060593A
Attorney, Agent or Firm:
AWEK INDUSTRIAL PATENTS LTD OY (P.O.Box 230, Lautatarhankatu 6, Helsinki, FI-00101, FI)
Download PDF:
Claims:

CLAIMS

1. A fastening screw (1) for a cylinder head of a piston engine, the fastening screw comprising two end sections (2), at least one of which is provided with threads, and a shank (3) between the end sections (2), characterised in that the cross-sectional shape of the shank (3) deviates from a circle.

2. A fastening screw (1) according to claim 1, characterised in that the cross-sectional shape of the shank (3) is oval.

3. A fastening screw (1 ) according to claim 2, characterised in that the ratio of the length (I) and width (w) of the cross-section of the shank (3) is 1.5 - 3, preferably 2.2 - 2.8.

4. A fastening screw (1) according to claim 1, characterised in that the cross-sectional shape of the shank (3) is quadrangular.

5. A fastening screw (1) according to claim 4, characterised in that the ratio of the length (I) and width (w) of the cross-section of the shank (3) is 1.5 -2.5, preferably 1.8 - 2.2.

6. A fastening screw (1 ) according to claim 1, characterised in that the cross-sectional shape of the shank (3) is triangular.

7. A fastening screw (1) according to claim 1, characterised in that the cross-sectional shape of the shank (3) is pentagonal.

8. A fastening screw (1 ) according anyone of the preceding claims, characterised in that the shank (3) is provided with elongated grooves O).

9. A fastening screw (1 ) according to claim 8, characterised in that the shank (3) is provided with three elongated grooves (9).

10. A fastening screw (1) according to claim 8 or 9, characterised in that the ratio of the depth (d) of the groove (9) to the radius (R) of the shank is 0.4 - 0.6.

11. A fastening screw (1 ) according to claim 8, 9 or 10, characterised in that the ratio of the width (w) of the groove (9) to the radius (R) of the shank is 0.8 - 1.

12. A fastening screw (1) according anyone of the preceding claims, characterised in that the cross-sectional area of the shank (3) is smaller than the cross-sectional area of the end section (2).

13. A fastening screw (1) according anyone of the preceding claims 1 - 11, characterised in that the cross-sectional area of the shank (3) is larger than the cross-sectional area of the end section (2).

14. A fastening screw (1 ) according anyone of the preceding claims, characterised in that the cross-sectional shape of the shank (3) is unsymmetrical with respect to the longitudinal axis (5) of the fastening screw.

15. A piston engine comprising an engine block (7) and a cylinder head (6) attached to the engine block (7), characterised in that the cylinder

head (6) is attached to the engine block (7) by means of fastening screws (1) according to anyone of claims 1 - 14.

Description:

FASTENING SCREW

The invention relates to a fastening screw according to the preamble of claim 1.

In engines, such as in diesel engines, the task of the cylinder head is to close the upper end of the cylinder liner and form the upper part of the combustion chamber. Cylinder heads are fastened in place by means of fastening screws. In order to ensure a gas-tight joint there has to be a sufficient number of fastening screws placed around the cylinder at as regular intervals as possible. The nuts of the fastening screws are tightened in an order and in phases given separately for each engine as well as with given tightening torques. In larger diesel engines the tightening of the nuts of the fastening screws for cylinder heads often takes place hydraulically, whereby the fastening screws will be provided with a precisely desired force.

The type of the fastening screw for a cylinder head is a so-called tension screw, which differs from an ordinary screw in such a way that its shank is for a given free length machined thinner than the screw ends. The length of this extension part is defined with respect to the dimensions of the thread so that, when tightening the screw up to the yield point of its material, the yielding occurs in the shank of the screw and the nominal stress in the cross-sectional surface of the thread is not allowed to exceed a permissible value. The shank of the screw is smooth-machined and the transition points are rounded with a suitable radius. The tension screws are manufactured of high-class alloy steels and they are used when high transition loads are exerted on the screw. A problem in the prior art solutions is the vibration of fastening screws during the engine

run and the damages to the screws due to the vibration. Similar problems also occur in other fastening screws used in piston engines.

An object of this invention is to provide an improved fastening screw.

The object according to the invention is achieved by a fastening screw according to claim 1, the fastening screw comprising a shank between end sections, the cross-sectional shape of which shank deviates from a circle.

Due to the cross-sectional shape of the shank deviating from a circle, the natural frequency of the fastening screw may be made unequal to, i.e. higher and/or lower than, the excitation frequencies occurring in a piston engine, especially the excitation frequencies occurring commonly in a piston engine that runs at medium speed. Consequently, the vibration of the fastening screw is small and thus also the damages caused by the vibration are minor.

In the following, the invention is explained by way of example with reference to the appended schematic drawings, of which

Figure 1 shows a fastening screw according to the invention;

Figures 2a - 2d show various embodiments of the fastening screw according to Figure 1 as a cross-section A-A;

Figure 3a shows a second fastening screw according to the invention;

Figure 3b shows the fastening screw according to Figure 3a as a cross- section;

Figure 4a shows a cylinder head for a piston engine attached to an engine block with a fastening screw according to the invention;

Figure 4b shows a cylinder head for a piston engine attached to an engine block with a fastening screw according to the invention;

Figure 5a shows a third fastening screw according to the invention;

Figure 5b shows a cylinder head for a piston engine attached to an engine block with a fastening screw according to Figure 5a.

Figure 1 shows one fastening screw 1 according to the invention for a cylinder head in a piston engine. The cylinder head of the piston engine is attached to an engine block by means of fastening screws 1. The type of the fastening screw 1 is a so-called stud bolt, i.e. its both end sections 2 are provided with threads. The cross-sectional shape of the end sections 2 is circular. A shank 3 is provided between the end sections 2. The shank 3 is unthreaded. The shank 3 forms a so-called extension part, whereby yielding occurs in the shank 3, when the fastening screw 1 is tightened up to the yield point of its material. Thus, the nominal tension in the cross-sectional surfaces of the threads of the end sections is not allowed to exceed a permissible value. The cross-sectional shape of the shank 3 deviates from a circle or deviates essentially from a circle. The shank 3 is solid. Between the shank 3 and both end sections 2 there is a transition area 4, by means of which the various shapes and sizes of the shank 3 and the end section 2 are joined together. The fastening screw 1 is a one-part screw. The fastening screw 1 is made, e.g., of austenitic steel. The length of the fastening screw 1 is typically 50 - 150 cm. Optionally, the fastening screw 1 may be provided with a screw

head, whereby one end section of the screw is provided with a thread and the other end section with a head, e.g. with a hexagon head (Figures 5a and 5b).

Figures 2a - 2d show preferable cross-sectional shapes of the shank of the fastening screw according to Figure 1. In these embodiments, the cross-sectional area of the shank 3 is smaller than the cross-sectional area of the end section 2. The cross-sectional shape of the shank 3 deviates from a circle or deviates essentially from a circle. The shape of the cross-section is constant over the entire length of the shank 3. The cross-sectional shape of the shank 3 may be unsymmetrical with respect to the longitudinal axis 5 (middle axis) of the fastening screw.

The cross-sectional shape of the shank 3 is such that the bending stiffness of the fastening screw 1 in various cross directions is unequal. In one direction, the bending rigidity of the shank 3 is smaller than that of a shank with a cross-sectional area of the same size and a circular cross-section. Then, the natural frequency of the fastening screw 1 is lower than the resonance frequency occurring normally in a piston engine. In another direction, perpendicular to the first direction, the bending rigidity of the shank 3 is higher than that of a shank with a cross-sectional area of the same size and a circular cross-section. Then, the natural frequency of the fastening screw 1 is higher than the resonance frequency occurring normally in a piston engine.

In Figure 2a the cross-sectional shape of the shank 3 is oval. The ratio of the cross-sectional dimensions, i.e. I (length) and w (width), is 1.5 - 3, preferably 2.2 - 2.8. The longitudinal axis 5 of the fastening screw 1 passes via the midpoint of the oval.

In Figure 2b the cross-sectional shape of the shank 3 is quadrangular, preferably rectangular. The short sides of the rectangle may be convex. The ratio of the cross-sectional dimensions, i.e. I (length) and w (width), is 1.5 - 2.5, preferably 1.8 - 2.2. The longitudinal axis 5 of the fastening screw 1 passes via the midpoint of the quadrangle.

In Figure 2c the cross-sectional shape of the shank 3 is triangular, preferably an equilateral triangle. The sides of the triangle may be convex. The longitudinal axis 5 of the fastening screw 1 passes via the midpoint of the triangle.

In Figure 2d the cross-sectional shape of the shank 3 is pentagonal, preferably an equilateral pentagon. The longitudinal axis 5 of the fastening screw 1 passes via the midpoint of the pentagon.

In the embodiment according to Figures 3a and 3b, the shank 3 of the fastening screw 1 is provided with elongated grooves 9, which are parallel with the longitudinal axis 5 of the screw. Elsewhere, the cross- sectional shape of the shank 3 is circular. There are three grooves 9 and they are arranged in angles of 120 degrees with respect to one another. The ratio of the width w of the groove 9 to the radius R of the shank 3 is 0.8 - 1. Similarly, the ratio of the depth d of the groove 9 to the radius R of the shank 3 is 0.4 - 0.6. In this way, the bending stiffness of the shank 3 will be high, whereby the natural frequency of the fastening screw 1 is higher than the resonance frequency occurring typically in a piston engine. In other respects, the fastening screw 1 is similar to the fastening screw shown in Figure 1.

Figure 4a shows a cylinder head 6 attached to an engine block 7 of a piston engine by means of a fastening screw 1 according to the

invention and a nut 8. The shank 3 of the fastening screw 1 is narrower than the end sections 2. One end of the fastening screw is screwed in the threads in the engine block 7. The cross-sectional shape of the shank 3 may be similar to the one shown in Figures 2a - 2d or 3b.

Figure 4b shows a fastening screw 1 , the shank of which is wider than the end sections 2. In other respects, the fastening screw is similar to the fastening screw shown in Figure 1 or Figure 3. The cross-sectional shape of the shank 3 may be similar to the one shown in Figures 2a - 2d or 3b. The cylinder head 6 is attached to the engine block 7 of a piston engine by means of the fastening screw 1 and a nut 8. The fastening screw 1 may be used, if the mounting order of the cylinder head 6 allows that.

Figures 5a and 5b show a fastening screw 1, one end section 2 of which is provided with threads and the other end section with a screw head 10. The cylinder head 6 is attached to the engine block 7 by means of the fastening screw 1.

All the above-described shapes of the shank 3 of the fastening screw may be provided by forging. The grooves 9 in the shank according to Figures 3a and 3b may also be provided by milling. The fastening screw 1 according to the invention is used especially in piston engines that run at medium speed. The rotational speeds of the engines of this kind are 300 - 1200 r/min. The natural frequency of the fastening screw 1 is smaller and/or higher than the excitation frequency/frequencies generally occurring in the engine. Thus, the vibration of the fastening screw 1 and the damages caused by it are minor. The fastening screw 1 may be used in piston engines also for other purposes than for attaching a cylinder head to an engine block, i.e., for instance as anchor

bolts that keep the engine block together. The fastening screw 1 may also be used in other machines and devices, in which vibration occurs at the natural frequency of a screw having a shank with a conventional circular cross-section, or in the vicinity thereof.