In many cases, metal fabrication involves a process of station-to-station handling of oily or slippery blanks. The most common method of handling such parts is by using suction cups connected to a centralised or decentralised vacuum source. Such cups are well suited to increasing automation in the press shop.

Yet challenges still remain. Suction cups can slip out of position or even drop parts due to oil or die lubrication used in the forming operation. If the cups slip out of position, the sheets will be incorrectly placed in the next station, resulting in a halting of production to manually reset the sheets. A dropped part can lead to even more disastrous consequences, such as scratched parts, line stoppage and required reset time. A further challenge is the continued technical developments and increasing speeds of automation, combined with the higher speed capabilities of metal fabrication machines, which will likely see 20-strokesper- minute press line times in the not too distant future.

This will impose great demands on the suction cups. It’s a factor many tend to forget, and if not taken into consideration, will make it impossible to take full advantage of the machine’s capabilities. A synchronized cross-bar press handling large body sides, for example, can be accelerated up to 30m/s2. These same pressings must be de-accelerated nearly as fast to be placed in the next station, while an emergency stop can cause even higher stresses on the suction cup-to-panel coupling.

What follows is a look at the best way to avoid slippage between suction cups and oily metal sheets, including a look at potential design pitfalls.

Theoretical analysis

A suction cup will lose position if the holding force/grip is lower than the sum of all counteracting forces. In very ‘slow’ handling applications, the ‘dimensioning’ force is that created by the weight of the object handled (and gravity). In faster applications, and especially those with rapid acceleration and/or deceleration, there are other important forces to take into consideration when dimensioning, that of the object’s mass, ‘m’, and the acceleration/deceleration/braking, ‘a’.

• Newton’s second law says: Facceleration/brake (N) = M x A.
• Where:
M = Weight of the object (kg)
A = Acceleration/deceleration (m/s2)

Suction cups generate a normal (vertical) lifting/holding force (Fn). They also generate a parallel friction force (Ffr). The level of the friction force depends on the friction coefficient (μ).

• Ffr = (Fn) x μ
Fn. = Vertical force created by the cup
μ = Friction coefficient between the materials in contact

The table below shows approximate and general values of the friction coefficient between the material combination of rubber and metal.

The friction coefficient is higher at rest (μrest) compared to motion (μmotion), meaning higher force is needed to start the motion than to maintain it. If cups start to slide due to an applied force, there is a greater risk that the cups will slide much more when the motion is started.

Handling a bonnet on a stamping line

A bonnet or hood is to be formed in a press line under the following conditions.

Application: First press stages
Blank size: 1000 x 1500 x 1mm
Oil on blank: 2 g/m2
Blank weight: 12kg
Suction cup: 75 mm diameter
Cup lifting force: 150N @ -60 kPa (Normal, vertical force)
Max parallel acceleration: 20 m/s2
Emergency stop: -30 m/s2
Max speed: 12 m/s
Press speed: 12 strokes per minute (720 parts per hour)

Step 1

Dimensioning is done by looking at the weight of the blank/ sheet and selecting a safety factor, a normal procedure for inexperienced designers. A typical safety factor of 2-4 will be used, and in this case we specify a minimum of safety factor of 4.
(i.e. 4x Fn x “safety factor” > Fweight x gravity.)

The weight of the blank is 12kg, so the designer calculates they will need four cups, each 75mm in diameter, which gives a safety factor of approximately 5. Is this enough to handle the acceleration/braking force?
• 4x Ffr must be > Facceleration/braking

It seems that four 75mm cups will be enough to handle an oily blank weighing 12kg, including a safety margin of 5. Step 2 takes a look at whether this is enough to handle the acceleration and braking forces.

Step 2

Here we take into consideration the maximum acceleration/ braking force (the ‘friction force’) that all the cups must withstand. By using Newton’s Second Law, the maximum acceleration of the machine will give a force of 12x20 = 240N. However, an emergency stop at maximum speed will result in a force of 12x30 = 360N.

In an emergency stop at maximum speed, suction cups with a total friction force of 360N are needed. The conventional cups we are using are made without any friction pattern. A handbook will show that the friction coefficient (μrest) is approximately 0.2 between rubber and oily metal.

The calculation results in 360/(150x0.2) = 12 cups, meaning a minimum of 12 conventional cups must be used to handle the emergency stop braking force. For an improved safety factor, 16 cups would be recommended.

Conclusions after Step 1 and Step 2

As Step 2 shows, the calculated four cups in Step 1 were far too few. Even under acceleration forces, the sheets would have moved. Rather than increase to 16 cups, which would necessitate rebuilding the end-of-arm tool, or add extra cups to the existing tool, the press line speed could be reduced.

The maximum speed of 12m/s generates emergency stop braking of -30 m/s2. To brake, the 12kg hood will create 360N in braking force. Assuming that four more cups can be added to the existing end-of-arm tool from Step 1, we have eight conventional cups with a total friction force of 150 x 0.2 x 8 = 240N.

The emergency stop deceleration/braking (A) can also be determined by:
A = (Vf – Vi) / t
Vf = Final speed
Vi = Initial speed
t = Time

A = -30 m/s2 (machine specification)
Vf = 0 (full braking)
Vi = 12 m/s (machine specification)

=> t = -12/-30 = 0.4 sec (time needed at maximum speed to completely stop)

If eight cups can handle just 240N, what is the maximum safe press speed? First, how powerful can the ‘emergency stop’ braking be with eight conventional cups?
240N/12kg => -20 m/s2

If we assume that the time (t) to achieve complete emergency stop braking is constant (0.4 seconds), this tells us that the maximum allowed speed (Vi) will be: 0.4 x 20 = 8m/s.

So the machine can only be run at a maximum speed of 8m/s and not 12m/s, if we are to manage a complete emergency stop without sliding cups, without 16 conventional cups on a rebuilt end-of-arm tool.

A 33% speed reduction during the vacuum handling cycle will definitely lead to a 20-25% reduction in total cycle time, meaning the machine will reach 9-10 strokes/min, rather than 12 strokes/min. Instead of 720 parts/hour, 600 parts/ hour would be more likely, resulting in a gross margin loss of approximately 120 parts/hour.

A press line usually runs 20-24 hours per day, meaning a daily output reduction of 2,400 parts when the line is run at 10 rather than 12 strokes/min due to the chosen cup solution. If each stamped hood has a gross margin value of 2, the gross margin loss will be €4,800 per day.

Step 3

There are tailor-made friction suction cups for oily sheets/ blanks. A state-of-the-art tailor-made friction cup can increase the friction coefficient (μrest) from approximately 0.2 to nearly 0.4-0.5, almost the same as on a dry metal sheet. Using the same calculations (with μrest = 0.45) as in Step 2, the result will be that only 6-8 friction cups are needed, including a safety margin to manage the 360N in emergency stop braking force.

Conclusions after Step 3

Using friction suction cups, the end-of-arm tooling can remain the same, while production can run at the maximum machine speed, 12 strokes/min. An emergency stop will not make the sheets/blanks slide.

In this simplified example we have just looked at a parallel motion with acceleration and braking. A vertical or angled motion with acceleration/braking will add even more to the needed friction force of the cups due to gravity.

We have assumed a decentralized vacuum system. The vacuum pump/generator solution can also affect speed. A decentralized system is faster in most cases.

Friction cups - crucial components in high-speed sheet metal fabrication

Higher speeds and output in metal fabrication have led to a situation where the material handling system (in most cases a vacuum system) has become the bottleneck. Until recently, friction cup quality has not been of a sufficient standard, plus many users and machine builders are not aware of friction cups. Sometimes, the bottle neck can be solved by adding more or larger standard cups, but in most situations there is no room for these due to the shape of the sheet or blank.

As we have shown, well designed friction cups can more than double the shear force/friction grip on an oily metal sheet, with fewer cups required to achieve the necessary grip, allowing the press machine to run at maximum speed.

Choose the correct friction cup

Only a handful of suppliers offer friction cups. Most of these feature an internal friction surface (knobs and oil channels for instance). These cups initially return a good friction force/grip, however, normal usage will all too soon wear out the friction pattern, the cups then returning more or less the same performance as conventional cups.

DuraFlex Friction Cups offer a special friction pattern design that resembles that of (stud-free) winter/snow tires. The sharp, finely grooved lamellar structure is key to achieving a super grip. This design maintains friction performance over the life of the cup, with initial wear actually serving to slightly improve the friction grip.