Greenhouses, their construction and equipment . Fig. 74.—Chain system of operating ventilators. Nocolumns used. Fig. 75.—Rack-and-pinion system of operatingventilators 134 GREENHOUSES to the shaft as in the method of actuallyopening the ventilators. The chief advan-tage of this system lies in the fact that thereis less torsional or twisting strain on theshafting than when the usual method is em-ployed, and they are more powerful. Thechief disadvantage is that provision must bemade for giving the shaft several revolutions,while a half or two-thirds revolution is usual-ly sufficient with the mor
Greenhouses, their construction and equipment . Fig. 74.—Chain system of operating ventilators. Nocolumns used. Fig. 75.—Rack-and-pinion system of operatingventilators 134 GREENHOUSES to the shaft as in the method of actuallyopening the ventilators. The chief advan-tage of this system lies in the fact that thereis less torsional or twisting strain on theshafting than when the usual method is em-ployed, and they are more powerful. Thechief disadvantage is that provision must bemade for giving the shaft several revolutions,while a half or two-thirds revolution is usual-ly sufficient with the more common forms. Some practical growers claim that the rackand pinion device is very subject to wear andis a frequent cause of trouble. This is moreespecially true of the older forms of this fact that they are not generally usedwould seem to indicate that practical growersas a rule are not yet convinced of their super-iority, though they are now being installed insome large houses where it is necessary tooperate long runs. Quite frequently the hand wheel and gear-ing are fastened to the rafters o
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