GB2207462A - Mounting I.C. engine camshafts - Google Patents

Abstract

Bearing surfaces 26 for the camshaft 16 are integral with the rocker cover 28. Half bearings 26 are used at all bearing surfaces, except the front bearing adjacent the input drive in the case of a belt or chain driven camshaft, and this also simplifies machining since a cutter can be used instead of an unsupported drill bar. <IMAGE>

Description

INTERNAL COMBUSTION ENGINE The present invention relates to the construction and assembly of internal combustion engines.

In a conventional overhead camshaft (OHC) engine, the camshaft is journalled in bearing blocks in the cylinder head. A cover is provided for the valve gear and serves to seal off the valve mechanism so as to prevent dirt from entering the engine and to contain the engine lubricant.

The cover is not a stress bearing component and can be manufactured from narrow gauge metal.

The disadvantage of the conventional construction is that a large number of components are involved and this in turn leads to a longer assembly procedure which increases manufacturing costs.

With a view to mitigating the foregoing disadvantages, the present invention provides an internal combustion engine having at least one overhead camshaft which is supported from above by means of bearing surfaces integral with a cover which overlies the valve mechanism and sealingly engages the cylinder head.

It should be mentioned here that the term cylinder head is not intended to signify necessarily a head separable from the cylinder block but the head may be an integral part of the engine block as in the case of a monoblock engine.

If the bearing caps and the cover used in a conventional engine are integrated into a single piece item certain problems arise and these problems will now be discussed.

In the first instance, line boring of the bearings cannot be used as one cannot steady the long drill used for boring. Therefore, larger clearances must be adopted to contain the resultant boring misalignment.

A second problem that arises is that one cannot have metal to metal contact at both the cap faces and the sealing flange. A relatively expensive flexible gasket is therefore necessary to seal the valve cover.

An analysis of the forces acting on the camshaft indicates that the loads acting on the bearing caps, excepting the bearing adjacent the input drive, are predominantly upwards, that is to say away from the cylinder head. The bearing adjacent the input drive has a downward component in the case of a belt or chain drive though in the case of a gear drive the forces would once again act upwards.

In the preferred embodiment of the invention, the camshaft is supported by half bearings over at least the parts of its length spaced from the input drive. The camshaft may be journalled in a fully circumferential bearing adjacent the input drive though in the case of a gear driven camshaft all the support surfaces may be half bearings.

The fact that most of the support surfaces are half bearings considerably simplifies machining, as the surfaces can now be cut using a stiffly supported cutter as opposed to a relatively unsupported boring bar. A small amount of overhang on the cutter permits the front fully circumferential bearing to be machined without increasing clearances.

The cover of the preferred embodiment of the invention can thus be manufactured simply without the problems mentioned above, since an accurate camshaft half bore can be formed in the cover in correct alignment with the sealing flange.

The cover may be bolted down onto posts straddling the bearing supports so that the stress of the valve springs should not be taken up by flexing of the cover.

Prior to mounting of the cover, the camshaft cannot be used to compress the valve springs and setting the valve clearance is therefore difficult. This problem may be however be avoided by the use of hydraulic self adjusting tappets, which are known per se.

The invention may be used either with rockers having tappets acted upon from above by the camshaft or in engines where the camshaft acts directly on cam follower buckets located in recesses above the valve stems. Where the camshaft acts on the valves by way of rockers, it is preferred that the tappets on the rockers be roller tappets.

If the rockers are mounted on a common rocker shaft it is found that the forces on the rocker shaft act predominantly downwards, that is to say towards the cylinders. It is in this case advantageous to mount the rocker shaft in a half bore machined by cutters in post projecting from the cylinder head so that once again machining is simplified and made more accurate and the assembly time is reduced.

In assembly of the engine of the preferred embodiment of the invention, the rocker shaft (complete with rockers and any spacing springs and washers that may be fitted onto the rocker shaft) is placed to rest in a half bore machined in posts projecting from the cylinder head with the rockers engaging the valve stems. The camshaft is next passed through the fully circumferential bearing in the cover and is supported along its length on the half bearings. After placing of a suitable gasket on the cylinder head, the rocker cover complete with the camshaft is lowered onto the cam followers and the rocker cover is tightened by means of bolts passing through the cover into the cylinder head.

The invention will now be described further, by way of example, with reference to the accompanying drawing which is a schematic section through an engine cylinder head.

The cylinder head compartment of the OHC engine shown in the drawing comprises inlet valves 10 and exhaust valves 12 which project from the cylinder head 14. The engine comprises a single overhead camshaft 16 which actuates both sets of valves 10 and 12 by way of respective rockers 18 and 20 fitted with roller cam followers 22 and mounted on a common rocker shaft 24. As so far described, the engine is generally conventional and it is believed that the operation of the engine need not be described in further detail.

In the prior art, the overhead camshaft is journalled in fully circumferential bearing comprising a bearing block fitted with a removable bearing cap. Several individual bearing caps need to be tightened over the camshaft at intervals along its length and the rocker cover is a further item which must be bolted onto the cylinder head.

Still further means are used for bolting the rocker shaft on the cylinder head.

In the illustrated embodiment of the invention, the rocker shaft 24 rests on posts 30 formed with semi-circular aligned half bores 32. The camshaft 16 rests in half bores 26 which are machined in a stiff rocker cover 28 held onto the cylinder head 14 by means of bolts 36. If required, further bolts may be provided adjacent the bearing surfaces 26 to prevent flexing of the rocker cover 28 under the action of the forces exerted by the valve stems. At the front end of the engine, adjacent a drive chain or drive belt for the camshaft, the cover 28 includes a machined fully circumferential bearing and the camshaft has one bearing surface received within this first bearing. All the remaining bearing surfaces supporting camshaft are half bearings, as illustrated.

In use the forces acting on the rocker shaft 32 are predominantly downwards and the forces on the camshaft 16 are predominantly upwards, as shown. The exception to this is that the camshaft is subjected to a downwards force near the drive end and it is for this reason that a fully circumferential bearing is provided near one end of the rocker cover 28. It is not therefore necessary to provide any more than half bearings in most locations and this is used in the present invention to simplify construction and assembly.

The construction is simplified in that instead of using a boring tool to form aligned holes for the support bearings, the half bearings can be machined by means of a rigidly supported cutter. It is not therefore to assemble bearing caps for machining and separate them for insertion of the camshaft and the accuracy of the bores is increased by the rigidly supported cutter. The same cutter can be used for the one fully circumferential bearing that may be required at the front end of the engine.

To assemble the illustrated engine top end, the rocker shaft 24 is mounted together with its rockers 18 and 20 on the posts 30 and the rockers are aligned with the valve 10 and 12. The valves have hydraulic tappets which are known per se and are self adjusting. A gasket is fitted to a flange 42 which projects from the cylinder head 14 and after placing the camshaft 16 on the support bearings 26 in the rocker cover 28, the latter is lowered onto the flange 42 and bolted down by means of the bolts 36.

It will be clear that the invention can also be applied to engines which have two overhead camshafts and to engines where the camshaft acts by way of cam follower buckets on the valves instead of by way of rockers.

Claims (6)

1. An internal combustion engine having at least one overhead camshaft which is supported from above by means of bearing surfaces integral with a cover which overlies the valve mechanism and sealingly engages the cylinder head.

2. An engine as claimed in claim 1, wherein the camshaft is supported by half bearings over at least the parts of its length spaced from the input drive.

3. An engine as claimed in claim 1 or 2, wherein the camshaft is journalled in a fully circumferential bearing adjacent the input drive.

4. An engine as claimed in any preceding claim, wherein the bearing surface in the cover are machined by means of a firmly supported cutter.

5. An engine as claimed in any preceding claim, wherein the valves are provided with hydraulic self-adjusting tappets.

6. An engine constructed arranged and adapted to operate substantially as herein described with reference to and as illustrated in the accompanying drawing.