Early in the nineteenth century, pocket clips appeared for men’s pens, originally as optional extras. Ladies often opted for smaller pens without a clip but having a ring at the cap’s top for attachment to chains. Some men used the same design and linked them to their watch chains and wore them in vest pockets.
Obviously, a good reason for a clip on a cap is that it causes the user to store the pen in a nib upwards position. This permits the possible air pressure increase in the reservoir to escape without pushing out any ink. Furthermore, it prevents the pen from being tossed around in a briefcase or getting damaged.
Styles of Clips
1.) Stiff Metal Clip, hinged, springloaded
Clips come in many versions. A good clip would be stiff within itself (so it can’t be damaged through deformation), be tiltably mounted on a pivot pin or edge and be pushed against the cap body with a return spring. See photo 1. These clips move relatively easy and can open up to five millimetres with the force hardly increasing. Later, further downs, I summarise the measured data in table 1.
An example of a well-ingeneered clip is shown in photo 2. Here, the clip is hinged on an actual hinge pin while in other design solutions of photo 1, the clip hinges on a sharp edge.
You may think that these are quite a few components to simply achieve a moveable clip. However, it is here where quality shows. Keeping the cost down depends on the skills of the manufacturing ingeneers. “Good quality does not have to cost one cent more,” is one of my favourite sayings.
In photo 1 and 2, the clips are solid but sufficient stiffness can also be created through the clip having a U, V or box-shaped cross-section. Photo 3 shows an example of this well-known construction.
Where the clip is attached to the cap body the highest moment occurs. There the V-shaped “feathers”, the clip still has some elasticity. The tubular shaft possesses the strongest stiffness. The point is formed hollow. The ring is embedded in an elastic mounting, which supplies the preload for holding the clip down and some lesser force for lifting the clip.
Disappointed I must add that a deceptively similar clip is used on Parker ballpoint pens where it is immovably connected with the pen body and all elasticity is permitted by the “feathered” section, which requires a high force to be overcome when lifting the clip. In such a situation a clip’s purpose is reduced to that of a mere ornamental logo to entice customers.
A similar example is shown in photo 4. Through profiling the cross-section, the clip is very stiff and neither provides the mounting with any elasticity. I marked the area of possible flexibility with a blue line. The same applies to the clip in photo 3. They show a style of construction I hardly recommend, the “bad” Parker and this one in photo 4 don’t belong under this heading, but where else?
2.) Stiff Metal Clip, flat spring/hinge
In the version of a clip shown in photo 5, the stiffness is created through a metal strip with a U-shaped cross-section. The clamping force between the clip-point and the cap body is caused by a blade spring, which also acts as the hinge.
Photo 6 shows a style of a clip applying the same principle, with an added feature which allows lifting the clip-point off the cap body when one pushes down on the extended end of the clip.
This helps the writer to slip the clip on and off onto thicker material because then, the closure force would be the highest. I have seen such a clip on a ball-pen where it is used to retract the refill. The same idea could be applied to a fountain pen cap to disengage the cap from the fountain pen. That would be something novel.
However, because they don’t offer any visible clue for their mode of separation, screw-on and clip-on caps cause already enough confusion, this third style would only add to the chaos. Let’s keep on thinking!
3.) Elastic, flexible Clips, flat spring
Elastic clips are flexible within themselves. Often they are flat and this way they offer the best, widest elastic characteristics and range.
A typical clip is shown in photo 7. Due to being work-hardened, the two bends are stiff, therefore the contour near the mounting with the cap body is largely inflexible. The bend angle is less than 90° which causes some interference between the two components. Hence, when assembled a preload is created between the cap body and the clip point. The clip is held in place by the red coloured component, which is pushed in after the clip is located, and it acts as a wedge. It further indicates the colour of the ink in the pen.
The clip shown in photo 8 has a curved cross-section which stiffens its shank.
Through the nature of a leaf spring, the resistance against a bending force increases rapidly with the reduction of the bending radius and most such small radii end up being work-hardened during forming, anyway. Where is this clip meant to bend? The only place left is the short flat section between the small radius and the cap body; it won’t be of much use and I easily can envisage the damage to the plastic ring which fixes the clip to the cap.
Therefore, the range of gentle flexibility of these types of clips is narrow. Forcing them on thick material may result in plastic deformation with the consequence of them not returning to the clip surface. I have talked about this principle in the chapter about Fountain Pen Nib Mechanics.
Since their clamping force is quite high when pushed on thicker material, they often mark or even damage this material.
In photo 9 I show an example of a flat spring clip. The clip point is formed by folding back the metal strip. Often such components are formed while the material is in its malleable state, and afterwards, they have been heat-treated to harden.
4.) Elastic Clips, Wire
From a construction and functional point, the only difference between this style in photo 10 and those in section 3.) is that the clip is made of a piece of elastic wire. Generally, the latter is not as useful as the flat spring clips because wires have a smaller elastic range when bent and therefore rather tend to show plastic deform more easily.
In my opinion, wire clips are more a trendy styling feature rather than good ingeneering design. It is the element of function, which draws the line between lasting pristine simplicity and fashionable decor. Photo 10 clearly exposes a gap between the cap body and the clip point after plastic deformation of an ill-designed clip. Further, it demonstrates that even a reputable brand name does not always stand for good ingeneering design but succumbs to superficial cosmetics, just like all the others sometimes do.
The clip shown in photo 11 is very stiff in the section of the wire but at least has some inbuilt elasticity in the clip mounting.
An example similar to the design in photo 11 is shown in photo 12. Here the elasticity is created by a (hopefully) elastic enough small leave spring on the mounting ring, which also serves to hold the wires in their position, rather than relying on the strength of the clip screw.
5.) Elastic Clips made of Plastic, two Parts
Here again, the construction principle is the same as the flat metal clips described in paragraph 3.), however, the clip is made from plastic with a certain elastic range of deformation. The cap body and the clip are two separate components often made of different materials of which the clip material is more flexible, see photo 13. The preload is achieved during assembly through the dimensional interference of the two components.
In comparison with metal, the elasticity range of plastics is considerably less than that of metal. Plastics reach plastic deformation at smaller openings between clip and cap body. You know what happens when one progresses beyond the range of elasticity. You also have heard me mention the creep of plastic when it is under constant load. In this design principle, it means that the clamping force reduces to hardly anything, eventually.
6.) Elastic Clips made of Plastic, one Part
The simplest and cheapest version of producing a clip is to injection mould the clip with the cap as one part, see photo 14. I have shown this construction for the sake of completeness. I have not seen it applied in fountain pens.
This construction produces no preload, there is always a gap between clip and cap body, even though it may be kept very narrow through clever mould construction. From experience, despite the fact that it depends on the choice of plastic material, this type of clip deforms very easily. They are suitable to hook onto a sash or the like but only provide some holding force after being deformed by the material they are clipped onto.
There are designs where the pressure point acts together with the pen barrel, as in photo 15. Initially, when the pen is inserted into the cap, they don’t quite line up, which they will when forced to completely engage. This causes a preload between the clip point and the pen barrel.
After some wear and tear, a constant battle between the force that keeps the cap on the pen and the force between the clip and whatever it is holding onto. At worst, the clip attachment loses and separates from the barrel.
The clip shape shown in photo 14 has a long lead-in before the pressure point comes into action which I consider an advantage when engaging the clip with some material because the clip may not be pushed off that easily from the barrel. Of course, it all depends on the construction parameters.
It amazes me that some upper-end brands present cheap and nasty clips on their pens and still charge exorbitant prices. Why they squeeze the last cent out of their products and sacrifice function and durability is beyond me. Is it because they get away with it? Because no one complains? I emphasise this even more because other manufacturers offer well-designed solutions, even in moderately priced, even cheap pens.
Enough of that… let’s talk about a good clip. Foremost, it has a “smooth” pressure point (contact with the cap body) so it can glide onto most surfaces with a predetermined, narrow range of friction.
Most clip designs show their pressure point-shaped through a small, fixed protrusion on the clip, the clip point. It can be that simple. Above that, we find even cheap pens’ clips with a free-spinning roller or ball at the clip point. Even if such a clip is heavily preloaded, it will still slide well. The ultimate design would offer free moving pressure surfaces on both sides. This way, one can determine the friction independent from the surface of the materials the clip is engaged with.
Whatever clip design, when the cap is on the pen, it prevents the pen from rolling off the desk. For some designs, this is its only function.
Here are some comparative data of clips, of several pens I found in my drawer. They are all of the hinged style because it is the only I want to consider. I have chosen three clips. The selection criteria for the pens? As I said, anything I found in my drawer, and what I could have coped with if it got damaged in the process, perhaps not the Montblanc because it has a particular sentimental value.
The first test group is of the style of a hinged metal clip as in photo 1 which has a stiff but hinged clip and includes a spring to cause the closing force like described in paragraph 1).
The second group is of the spring metal style which has a stiff clip made of one material, often a die-cast, while the hinge is replaced by a blade spring. In both tested versions, the blade spring is attached to a clip mounting ring as shown in photo 5 in paragraph 2).
The clip of the springy plastic style as described in paragraph 5) is the third style of which I only found but one. The clip and the cap are two components and the preload is caused by a dimensional interference during assembly. Unfortunately, after time, plastics creep hence, the preload reduces to basically null.
I held the cap horizontal and applied a load near the clip point (see photo 16). Three measurements were taken; first the preload just at lift-off (when a 0.05mm slip-gauge just became “slippable”), then at 0.7mm (a credit card) and at 2mm (a perspex ruler). The results are collated in table 1.
|1||Lamy cp1||Hinged metal||36||1.4||1.7||2.4||0.5|
|2||Lamy 2000||Hinged metal||30||0.7||0.9||1.2||0.25|
|3||Shorty||Hinged metal||20||0||0.8||2.2 c||1.1|
|4||Montblanc 22||Spring metal||32||2.5||4.6||8.5 c||3|
|5||Curved wire||Spring metal||50||0||1.1||3.2||1.6|
|6||Sharpie||Spring plastic||35||0.5||2||4.6 c||2.1|
The red c indicates that I calculated the value to avoid damage to the clip. And remember: 1N (Newton) ≈ 102 grams, the weight of a bar of chocolate including the wrapper. That’s because Earth’s gravity is 9.80665m/s2 and not 1.000000m/s2. I wonder who got that wrong?
Discussion: The desired feature from a clip is that we want the clip to provide a certain clamping force to hold the pen securely. Preferably, the force should be effected as little as possible by increasing clamping thickness.
Hinged clips are the only construction a responsible ingeneer would consider to offer their users.
1.) Indubitably as well as expected, the hinged and spring-loaded metal clips perform the best and I like to recommend the “Lamy cp1” as a benchmark. You can see the small spring constant, which indicates their softness. Combined with a preload, their clamping force does not change much (≈ 30%) when increasing the gap.
2.) The “Lamy 2000” is a bit too soft. A stronger compression spring could have improved the performance.
3.) The “Shorty” having no preload has a strong increase of force, from 0.7mm up to 2mm almost threefold. Its clip was too stiff for the 2mm gap to be practical. I found this clip useless, merely ornamental.
All clips of the second group, did not close in their rest position, leaving gaps of 0.4mm up to 1mm.
4.) Montblanc offered a gold-plated ornament but not a clip. Its high spring constant takes it outside the practical range; to serve as a useful clip it is far too stiff. I didn’t dare open it to 2mm because I worried it would deform permanently or even break. I used the preload and 0.7mm values to calculate the value for the 2mm gap.
5.) For the test with a curved wire clip, I used the example by Shaeffertec as in photo 15. For a flat spring clip, it had the lowest spring constant in this group, mind you, it also had by far the longest clip.
The plastic spring clip I only included here for completion’s sake. I am glad that I was never asked to construct one… I always found someone else to do it.
6.) Sharpie’s clip was too stiff for the 2mm gap to be practical.
Concluding, we can all safely agree that clips and their construction have been neglected. A fruitful designer/ingeneer team could find a wide field open for their creativity to prosper.
Above all: Enjoy!
16 January 2019
go to Homepage and Content