Hello tsmith7393, many resistance spot welders use pneumatically controlled cylinders to apply the pressure as hydraulic cylinders are generally reserved for items that require greater amounts of pressure. The automotive and truck industry uses a great number of robotic type spot welders and I don't recall if they are hydraulically clamped or pneumatically clamped. Depending on the industry application they may also use manually actuated clamping mechanisms that are operated by hand or possibly by a footpedal, if there are units out there that use hydraulics specifically I'm sure that other posters will make you aware of them. Regards, aevald