Indexed quick-release rail mount for repositioning a vibration sensor on industrial machinery
F16M 13/02 Other supports for positioning apparatus, for attaching to an object
G01H 1/00 Measuring vibrations in solids by direct conduction to the detector
A vibration sensor is mounted on a carrier that slides along a short dovetail rail fixed to the machine. A spring-loaded index pin in the carrier drops into one of a row of holes in the rail, so the sensor returns exactly to defined measuring positions, and a cam lever then clamps the carrier hard against one flank of the dovetail to remove all play. Moving the sensor to another position takes about ten seconds instead of loosening and retightening several screws, while the clamped mount stays stiff enough not to distort the vibration signal.
- Stud-mounted and magnetic accelerometer mounts (general technique)
- Dovetail slides with clamping levers (common in machine tools and optical mounts)
Problem
In condition monitoring, an engineer often has to move a vibration sensor between several positions on a machine, for example along a gearbox housing. With a conventional bracket, this means loosening several fasteners, moving the sensor, finding the old position again by eye and retightening, which takes minutes and makes readings hard to compare over time. Simple sliding or magnetic mounts are fast, but they have play or low stiffness, which adds resonances and distorts the measured vibration, especially at higher frequencies.
Detailed description
The mount has a base plate (40), a dovetail guide rail (42), a sensor carrier (44), a row of index holes (46), a spring-loaded index pin (48), a clamping cam lever (50) and a sensor stud (52).
The base plate (40) is steel, 10 mm thick, and is fastened to a flat machined area of the machine frame with two M8 bolts (or two welded studs where drilling is not allowed). The rail (42) is a 60 degree dovetail, 120 mm long and 20 mm high, screwed to the base plate or machined in one piece with it. On its top face it has seven index holes (46), 6 mm in diameter and 15 mm apart.
The carrier (44) is a steel block with a matching dovetail groove and a sliding clearance of about 0.05 mm. The index pin (48) sits in a bore in the carrier above the row of holes and is pressed down by a compression spring with a force of 20 to 40 N; a knob on top lifts it. The cam lever (50) is mounted on one side of the carrier. When it is closed, its cam pushes a pressure pad against one flank of the dovetail, pulling the carrier against the opposite flank and against the rail top. This removes the sliding clearance, so the carrier, rail and base behave as one stiff part.
The sensor is screwed onto the stud (52) on the carrier's top face. The face around the stud is spot-faced flat, wider than the sensor base, so the sensor sits on full metal contact.
To move the sensor: open the cam lever, lift the index knob, slide the carrier, release the knob near the new hole so the pin drops in, then close the cam lever. The pin sets the position; the cam provides the stiffness. Measurements are only taken with the cam closed.
Example
On a gearbox with a 0 to 2 kHz measurement range, the mount was bolted to the housing next to the bearing seats. With the cam closed, a tap test on the carrier showed the first mount resonance at about 7 kHz, well above the measurement range; with the cam open it dropped to about 1.5 kHz, inside the range, which is why measurements are only taken with the cam closed. Moving the sensor between positions 1 and 7 took 8 to 12 seconds, compared with about 3 minutes with the previous four-screw bracket, and repeat readings at the same position differed by less than 3 % in overall vibration level.
Variants
Instead of a dovetail, a T-slot rail with a T-shaped carrier foot can be used; the cam lever then pulls the foot up against the underside of the slot lips.
Instead of a cylindrical index pin, a spring-loaded ball can engage conical recesses. This is faster to move but locates less firmly, so the cam clamp is still needed before measuring.
Instead of the cam lever, a quarter-turn screw with a pressure pad can clamp the carrier, for places where a protruding lever could catch on moving parts.
Two rails can be crossed on one base plate, with a second carrier on the first, to position the sensor in two directions.
Where the sensor must be electrically insulated from the machine, a thin hard-anodised or ceramic-coated pad can be placed under the sensor base; a soft rubber pad is not used, because it would add the resonance the clamp is meant to avoid.
Drawings
Figure 1 shows the mount from the side. The base plate (40) sits on the machine frame (hatched line) and carries the dovetail rail (42). The short vertical marks on the rail are the index holes (46). The carrier (44) sits on the rail; the index pin (48) with its knob is on the right of the sensor, and the cam lever (50) on the left. The sensor is screwed onto the stud (52).