single acting with and without spring return
Hydraulic work supports are used to provide a self-adjusting rest for workpieces and avoid their vibration and deflection under machining loads.
To reduce the dimensions and the expenses for the control, most of the work supports are single acting with spring return of the support plunger. Double-acting elements offer the advantage that the support plunger can be returned to the off-position within a precisely defined time.
For the support plunger there are 3 types of operation:
The support plunger is pushed in by the workpiece against the spring force.
The support plunger extends pneumatically and returns by spring force.
The support plunger extends hydraulically with spring force and returns by spring force or hydraulically.
The workpiece must not be deformed by the contact of the support plunger. Therefore the contact is made with spring force or pneumatically. Depending on the size, the spring forces are between 8 and 100 N.
The spring force is the smallest, when the distance between contact bolt and workpiece is the largest.
With pneumatic operation the contact force can be precisely adapted by a pneumatic pressure reducing valve. The pneumatic connection serves at the same time as connection for positive air pressure protection.
The work supports function in any mounting position.
The technical data are only valid for the vertical mounting position. Due to the low spring forces the weight of the support plunger and the contact bolt can influence the contact force and speed.
All work supports are equipped with wipers for the support plunger. Some versions have a metallic wiper edge to protect the wiper against rough and hot swarf.
To bridge the clearance between support plunger and locking bush an oil pressure of 25 to 50 bar is required. The minimum operating pressure of 100 bar already guarantees remarkable holding forces.
Work supports can only compensate the maximum load force with the maximum operating pressure.
The admissible load forces with other pressures can be taken from the load force diagrams on the data sheets.
If the forces indicated in the load force diagram are exceeded by more than 10 %, the support plunger can yield.
Work supports compensate only forces in the direction of the plunger axis.
If side loads will be introduced into the support plunger, the thin-walled locking bush deforms so that the exact position of the workpiece is no longer guaranteed.
That means that the side loads generated during machining have to be compensated by fixed workpiece supports or by horizontal stops.
The safest protection against penetration of liquids and particles is the connection of positive air pressure protection. This is possible for all work supports with vent port.
The air pressure must not exceed 0.2 bar.
Pneumatic position monitoring is possible for work supports as per data sheet B 1.930.
Work supports have such as other steel components an elastic behaviour, i.e. they yield in case of load. The below diagram shows the elastic deformation of a work support with load. The comparison with a threaded screw jack used for support shows better results for the work supports.
In the body of the work support a thin-walled locking bush is integrated, which locks cylindrically around a movable support plunger when pressurising the element with hydraulic oil. This function requires a minimum operating pressure of 100 bar.
After depressurising, the support plunger is again freely-movable.
In the body of the work supports an additional locking piston is installed that locks a movable support plunger when pressurising the element.
After depressurising, the support plunger remains locked.
For unlocking a second hydraulic port must be pressurised, i.e. this element can only be operated in the double-acting mode.
The positions "support plunger retracted" and "support plunger extended" can be monitored
with the pneumatic position monitoring. The pneumatic ports are directly at the housing of the work supports.
Support plunger: stainless special steel, hardened
Locking bush: stainless high quality steel
Body: free-cutting steel, black-oxide
Interior parts: nitrocarburized or stainless
Seals: NBR or FKM
NBR = nitrile-butadiene rubber trade name e.g. Perbunan operating temperature: -30 to +100 °C
FKM = fluoro rubber trade name e.g. VITON® operating temperature: -20 to +200 °C
All work supports, where the support plunger contacts the workpiece by spring force, must be vented (exception B 1940). Especially when using coolants a venting port is imperative to avoid that the coolants will be sucked in into the interior of the work support. In the case of non-compliance troubles of functioning can occur.
If the flow rate is too high, the oil pressure increases so quickly that the support plunger will be locked before contacting the workpiece. In the case of this malfunction the flow rate has to be throttled.
The admissible flow rate is indicated on the corresponding data sheet.
If several work supports are connected to one line, the admissible pump flow rate is the total of the individually admissible flow rates:
V adm. pump ≤ n * V adm. support.
If the pump flow rate is higher than the admissible flow rate for the work support, throttling has to be effected in the supply line.
Air in the oil can considerably prolong the clamping time. Work supports require only a very small oil volume for their operation. Since the hydraulic oil in the connecting line is nearly not moved, careful
bleeding is required. If bleeding screws are not available, in the case of drilled channels screw plugs should be provided at the highest and remotest point.
Attention! Bleed always at low pressure.
Most of the work supports are delivered with a contact bolt in the support plunger.
This contact bolt has a dome-head and hardened contact surface.
Attention! Never use the work support without contact bolt, since penetrating dirt and liquids impede the function.
When using special contact bolts the following has to be considered:
1. The contact surface shall be hard and slightly dome-head so that it contacts safely the workpiece even in the case of uneven surfaces.
2. A plane contact surface can only be realised with a swivel contact bolt. However you have to reckon with a higher elastic compliance with load, since the swivel contact bolt will yield.
3. A contact bolt with point or ribbing has the disadvantage that the points press into the workpiece with load which results in a higher elasticity. In addition, there will be a form fit so that side loads will be introduced into the work support, which is not admissible.
4. The threaded stem of special contact bolts must have the same length and interior contour as the original. On demand a drawing can be sent.
5. Special contact bolts should have a maximum weight of 100 grams to guarantee the springy contact and the return stroke of the support plunger. Please contact us for the use of heavier or longer contact bolts.
The admissible load force of work supports has always to be dimensioned so that the clamping force of the used
clamping elements and the static and dynamic machining forces can be safely compensated.
- clamping force
- safety (reserve)
= possible machining force:
If the total of all occurring forces exceeds the admissible load force, the support plunger of the work support will be pushed back and the work support will be damaged.
The admissible load forces as per the diagram are static. The machining forces can also generate vibrations which exceed by far the mean value. For this reason a corresponding safety factor has to be taken into account.
The sequence – supporting and clamping – has to be controlled as a function of the pressure, e.g. by a sequence valve. If due to a too high flow rate a throttle valve is required, installation should be made as shown in the hydraulic circuit diagram.
If not otherwise indicated, the general tolerances as per DIN 7168 page 1+2 are valid for the connecting dimension.