in a faster time
By James de Havilland
IMPROVED bale wrapping times and more consistent wrap are among advantages claimed for the powered film pre-tensioner soon to be introduced by Tunbridge Wells-based KO Pressure Frames.
Developed over the past three years, the system uses hydraulically driven rollers to pre-tension the film. Existing pre-tension systems use the rotation of the bale to draw film through non-driven rollers.
Powering the rollers to stretch the film enables different rates of stretch to be selected, 40%, 70% or 130% currently being offered. With a conventional system, only one percentage can usually be set, with 70% being typical in the UK.
Second, the powered system provides a consistent stretch, with the film being fed to the bale at a rate hydraulically linked to the wrapper turntable speed. Rather than let the temperature-sensitive elasticity of the film determine the degree of stretch, the powered rollers maintain a consistent tension regardless of turntable speed.
The third advantage claimed for the powered system is that the film is fed to the bale, which means the turntable can be set at a higher speed, as the force pulling film from the reel through a conventional pre-tensioner is eliminated.
"An existing bale wrapper will be set to run at between 17 and 24rpm. With the power pre-tensioner, the turntable can run at up to 55 rpm," says KOs Ken Orpen.
Although a faster operating speed is a key advantage of the powered pre-stretch system, Mr Orpen believes an ability to stretch film accurately is also important.
"To ensure a perfect seal, the film needs to be correctly overlapped and the film itself not over stretched," he says. "In hot weather, film being drawn through a conventional stretcher will tend to "neck" – narrow in width as it is drawn out. This means it may be stretched to the point where it becomes too narrow to give the correct degree of overlap."
Mr Orpen concedes it is possible on existing systems to compensate for temperature changes by slowing the turntable to reduce tension or by adding another complete cycle of the bale. "But in practice most operators either avoid wrapping on really hot days, add another couple of wraps or simply do not bother."
Most leading silage film makers guarantee their products when wrapping at four layers with a 50% overlap, but Mr Orpen maintains the only way to ensure this is to wrap a bale and then weigh the removed film to see if the correct quantity has been applied.
"A 1.2m (4ft) diameter bale wrapped with 18 turntable revolutions and a 750mm wide film at zero tension will use 1.5kg of film. If this exercise is repeated with 70% tension, applied correctly, then the corresponding weight of film should be 850g. If less weight of film is used when wrapping at 70%, then obviously not enough film has been applied and the bale may not be protected adequately.
"At the other extreme, applying too much film is wasteful, with poorly stretched film sticking to itself less effectively. This will be indicated if the weight of film used increases above 850g."
Mr Orpen advises the wrap and weigh system is used to set up a silage wrapper initially, including one equipped with his companys power pre-stretcher.
During development the power pre-tensioner system has been used to wrap bales with six layers of conventional film at a 25% overlap and stretched to 130%. The actual quantity of film used is the same as for four layers at a 50% overlap tensioned to 70%.
"The six-layered bales are superbly wrapped, with the film adhering extremely well. In the development trials we have had users say they have yet to have a poor bale when wrapped this way," says Mr Orpen. He claims the extra layers of wrapping do not compromise bale throughput and in Scandinavia, where six layers of film are the norm, film savings of up to 30% have been possible using the new tensioner.
KO Pressure Frames aims to have the powered film tensioner available commercially in 1996 as an option for all bale wrappers, although a few will be available this year. The unit is expected to add another £1000 on to the price of the wrappers. *